mbstring no longer provides 'long' substitutions for erroneous input bytes

Previously, mbstring had a special mode whereby it would convert
erroneous input byte sequences to output like

mbstring no longer provides 'long' substitutions for erroneous input bytes

Previously, mbstring had a special mode whereby it would convert
erroneous input byte sequences to output like "BAD+XXXX", where "XXXX"
would be the erroneous bytes expressed in hexadecimal. This mode could
be enabled by calling `mb_substitute_character("long")`.

However, accurately reproducing input byte sequences from the cached
state of a conversion filter is often tricky, and this significantly
complicates the implementation. Further, the means used for passing
the erroneous bytes through to where the "BAD+XXXX" text is generated
only allows for up to 3 bytes to be passed, meaning that some erroneous
byte sequences are truncated anyways.

More to the point, a search of publically available PHP code indicates
that nobody is really using this feature anyways.

Incidentally, this feature also provided error output like "JIS+XXXX"
if the input 'should have' represented a JISX 0208 codepoint, but it
decodes to a codepoint which does not exist in the JISX 0208 charset.
Similarly, specific error output was provided for non-existent
JISX 0212 codepoints, and likewise for JISX 0213, CP932, and a few
other charsets. All of that is now consigned to the flames.

However, "long" error markers also include a somewhat more useful
"U+XXXX" marker for Unicode codepoints which were successfully
decoded from the input text, but cannot be represented in the output
encoding. Those are still supported.

With this change, there is no need to use a variety of special values
in the high bits of a wchar to represent different types of error
values. We can (and will) just use a single error value. This will be
equal to -1.

One complicating factor: Text conversion functions return an integer to
indicate whether the conversion operation should be immediately
aborted, and the magic 'abort' marker is -1. Also, almost all of these
functions would return the received byte/codepoint to indicate success.
That doesn't work with the new error value; if an input filter detects
an error and passes -1 to the output filter, and the output filter
returns it back, that would be taken to mean 'abort'.

Therefore, amend all these functions to return 0 for success.

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Test behavior of 'long' illegal character markers

After mb_substitute_character("long"), mbstring will respond to
erroneous input by inserting 'long' error markers into the output.
D

Test behavior of 'long' illegal character markers

After mb_substitute_character("long"), mbstring will respond to
erroneous input by inserting 'long' error markers into the output.
Depending on the situation, these error markers will either look like
BAD+XXXX (for general bad input), U+XXXX (when the input is OK, but it
converts to Unicode codepoints which cannot be represented in the
output encoding), or an encoding-specific marker like JISX+XXXX or
W932+XXXX.

We have almost no tests for this feature. Add a bunch of tests to
ensure that all our legacy encoding handlers work in a reasonable
way when 'long' error markers are enabled.

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Mark CP932 and CP51932 encoding tests as 'slow tests'

Migrate more SKIPIF -> EXTENSIONS (#7139)

This is a mix of more automated and manual migration. It should remove all applicable extension_loaded() checks outside of skipif.inc files.

Convert U+00AF (MACRON) to 0x8150 (FULLWIDTH MACRON) in some SJIS variants

Except for vanilla Shift-JIS, where 0x7E is a halfwidth overline/macron.
As for Shift-JIS-2004, it has an added

Convert U+00AF (MACRON) to 0x8150 (FULLWIDTH MACRON) in some SJIS variants

Except for vanilla Shift-JIS, where 0x7E is a halfwidth overline/macron.
As for Shift-JIS-2004, it has an added character (byte sequence 0x854A)
which was defined as a halfwidth macron in JIS X 0213:2000, so we use that.

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Convert U+203E (OVERLINE) to 0x8150 (FULLWIDTH MACRON) in some SJIS variants

Converting U+203E to 0x7E was especially wrong for CP932, where 0x7E
represents a tilde.

For vanilla

Convert U+203E (OVERLINE) to 0x8150 (FULLWIDTH MACRON) in some SJIS variants

Converting U+203E to 0x7E was especially wrong for CP932, where 0x7E
represents a tilde.

For vanilla Shift-JIS and Shift-JIS-2004, converting to 0x7E is acceptable,
since 0x7E does represent an overline/macron in those encodings.

Follow the same principle in CP51932, which is closely related to CP932.

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0x5C is not a Yen sign in CP932 (or CP51932)

When Microsoft created CP932 (their version of Shift-JIS), they explicitly
used bytes 0-0x7F to represent ASCII characters rather than JIS X

0x5C is not a Yen sign in CP932 (or CP51932)

When Microsoft created CP932 (their version of Shift-JIS), they explicitly
used bytes 0-0x7F to represent ASCII characters rather than JIS X 0201
characters.

So when converting Unicode to CP932, it is not correct to convert U+00A5
to CP932 0x5C. Fortunately, CP932 does have a multi-byte FULLWIDTH YEN SIGN
character which we can use instead.

CP51932 uses the same extended character set as CP932; while CP932 is
MicroSoft's extended version of Shift-JIS, CP51932 is their extended version
of EUC-JP. So the same reasoning applies to CP51932.

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Enhance handling of CP51932 encoding

- Don't pass 'control' characters through in the middle of a multi-byte char
- Treat truncated multi-byte characters as an error