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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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+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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Test EUC-JP and Shift-JIS more thoroughly

Previously, the unit tests for these text encodings covered all mappings
from legacy -> Unicode, and all _reversible_ mappings from Unicode -> l

Test EUC-JP and Shift-JIS more thoroughly

Previously, the unit tests for these text encodings covered all mappings
from legacy -> Unicode, and all _reversible_ mappings from Unicode -> legacy.
However, we should also test the few Unicode -> legacy mappings which
are not reversible.

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Fix mbstring support for EUC-JP text encoding

- Don't allow control characters to appear in the middle of a multi-byte
character. (A strange feature, or perhaps misfeature, of mbstring

Fix mbstring support for EUC-JP text encoding

- Don't allow control characters to appear in the middle of a multi-byte
character. (A strange feature, or perhaps misfeature, of mbstring which is
not present in other libraries such as iconv.)
- When checking whether string is valid, reject kuten codes which do not
map to any character, whether converting from EUC-JP to another encoding,
or converting another encoding which uses JIS X 0208/0212 charsets to
EUC-JP.
- Truncated multi-byte characters are treated as an error.

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