tests: add and use `%PERL` variable to refer to the Perl binary

To ensure Perl invocations within tests call the same Perl binary used
for running the tests, as specified or auto-detecte

tests: add and use `%PERL` variable to refer to the Perl binary

To ensure Perl invocations within tests call the same Perl binary used
for running the tests, as specified or auto-detected via `PERL` env
(autotools) or `PERL_EXECUTABLE` setting (cmake). Instead of the first
`perl` executable found in `PATH`.)

Cherry-picked from #14949
Closes #15097

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misc: better random strings

Generate alphanumerical random strings.

Prior this change curl used to create random hex strings. This was
mostly okay, but having alphanumerical ran

misc: better random strings

Generate alphanumerical random strings.

Prior this change curl used to create random hex strings. This was
mostly okay, but having alphanumerical random strings is better: The
strings have more entropy in the same space.

The MIME multipart boundary used to be mere 64-bits of randomness due
to being 16 hex chars. With these changes the boundary is 22
alphanumerical chars, or little over 130 bits of randomness.

Closes #11838

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tests: use %LOGDIR to refer to the log directory

This will allow it be set dynamically.

Ref: #10818

tests: skip mime/form tests when mime is not built-in

Closes #9596

mime: use percent-escaping for multipart form field and file names

Until now, form field and file names where escaped using the
backslash-escaping algorithm defined for multipart mails.

mime: use percent-escaping for multipart form field and file names

Until now, form field and file names where escaped using the
backslash-escaping algorithm defined for multipart mails. This commit
replaces this with the percent-escaping method for URLs.

As this may introduce incompatibilities with server-side applications, a
new libcurl option CURLOPT_MIME_OPTIONS with bitmask
CURLMIMEOPT_FORMESCAPE is introduced to revert to legacy use of
backslash-escaping. This is controlled by new cli tool option
--form-escape.

New tests and documentation are provided for this feature.

Reported by: Ryan Sleevi
Fixes #7789
Closes #7805

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