Use PCRE for our fallback regex engine when regcomp_l is unavailable

Remote URL last-chance resolution

Explicitly enable the `builtin` regex backend and the PCRE backend for
some Linux builds.

Since libssh2 doesn't read host configuration from the config file,
this callback can be used to hand over URL resolving to the client
without touching the SSH implementation itself.

This avoids any misunderstanding with the REGEX keyword in cmake.

Skip UTF8 BOM in ignore files

We've already added `ZLIB_LIBRARIES` to `LIBGIT2_LIBS` so don't also add the `z` library

We've already added `ZLIB_LIBRARIES` to `LIBGIT2_LIBS` so don't also add the `z` library (libgit2/#5079).

Define SYMBOLIC_LINK_FLAG_DIRECTORY if required

Ensure that we can read and parse an ignore file with a UTF8 BOM.

Use PCRE2 and its POSIX compatibility layer if requested by the user.
Although PCRE2 is adequate for our needs, the PCRE2 POSIX layer as
installed on Debian and Ubuntu systems is broken, so we do not opt-in to
it by default to avoid breaking users on those platforms.

Attempt to locate a system-installed version of PCRE and use its POSIX
compatibility layer, if possible.

Users can now select which regex implementation they want to use: one of
the system `regcomp_l`, the system PCRE, the builtin PCRE or the
system's `regcomp`.

By default the system `regcomp_l` will be used if it exists, otherwise
the system PCRE will be used.  If neither of those exist, then the
builtin PCRE implementation will be used.

The system's `regcomp` is not used by default due to problems with
locales.

Move some win32 type definitions to a standalone file so that they can
be included before other header files try to use the definitions.

PCRE includes compatibility functions that may go unused.  Don't warn.

The '[[:digit:]]' and '[[:alpha:]]' classes require double brackets, not
single.

The regex functions return nonzero (not necessarily negative values) on
failure.

In order to avoid us being unable to match characters which are part of
the normal US alphabet in certain weird languages, add two tests to
catch this behavior.

In order to make it easier adding more locale-related tests, add a
generalized framework handling initial setup of languages as well as the
cleanup of them afterwards.

While we already have a test for `p_regexec` with `LC_CTYPE` being
modified, `regexec` also alters behavior as soon as `LC_COLLATE` is
being modified. Most importantly, `LC_COLLATE` changes the way how
ranges are interpreted to just not handling them at all. Thus, ensure
that either we use `regcomp_l` to avoid this, or that we've fallen back
to our builtin regex functionality which also behaves properly.

While the test asserts that the error value indcates a non-value, it is
actually never getting assigned to. Fix this.

When searching for a configuration key for the diff driver, we construct
the config key by modifying a buffer and then passing it to
`git_config_get_multivar_foreach`. We do not check though whether the
modification of the buffer actually succeded, so we could in theory end
up passing the OOM buffer to the config function.

Fix that by checking return codes. While at it, switch to use
`git_buf_PUTS` to avoid repetition of the appended string to calculate
its length.

Use PCRE 8.42 as the builtin regex implementation, using its POSIX
compatibility layer.  PCRE uses ASCII by default and the users locale
will not influence its behavior, so its `regcomp` implementation is
similar to `regcomp_l` with a C locale.

Use the system includes (defined by libgit2) as the fuzzer includes.
The fuzzers link against internal libgit2 API and therefore need to have
the full include path that libgit2 uses.

Prefix all the calls to the the regexec family of functions with `p_`.
This allows us to swap out all the regular expression functions with our
own implementation.  Move the declarations to `posix_regex.h` for
simpler inclusion.

Support symlinks for directories in win32

rebase: orig_head and onto accessors

This is not implemented and should fail, but it should also not leak. To
allow the memory debugger to find leaks and fix this one we test this.

If someone passes just one ref (i.e. "master") and misses passing the
range we should be nice and return an error code instead of crashing.