Update fuzz support to reduce JIT timeouts by limiting quantifiers on groups. Also some general tidies.

Fix incorrect compiling when variable-length lookbehind's first branch was not the shortest and there was a backreference to a groupwithin the lookbehind.

breaks `make dist` and while it might be tweaked further, is likely to
need time to stabilize so doing so has been punted for now.

This mostly reverts commit def175f4.

Fix incorrect matching of 0xffffffff to any character with more than one other case in 32-bit UCP (but not UTF) mode.

Fix compile loop in 32-bit mode for characters above the Unicode limit when caseless and ucp are set.

* pcre2posix: make code warning free and update ChangeLog

Somehow previous fix was not ammended to include this code change,
take the opportunity to update ChangeLog and do other cleanup so
it will be at least worth a PR.

Those found responsible have been sacked

* pcre2posix: fix crash on recent regerror code

Since 0710ce2b (pcre2posix: avoid snprintf quirks in regerror (#333),
2023-11-15), a call for snprintf was replaced by a pair of strncpy
and buf[errbuf_size - 1] = 0, but it didn't account for the case
where errbuf_size == 0.

Make the code conditional to mimic the original logic and avoid
crashing.

* doc: document that the POSIX interface is not POSIX compatible

POSIX 1003.1-2008 requires that regoff_t be at least as large as
ssize_t or ptrdiff_t, but we use int and therefore any match is
restricted to what that can hold, even in 64-bit architectures.

Fix misbehaviour of pcre2_match() and pcre2_dfa_match() when PCRE2_FIRSTLINE was set for an anchored pattern.

While at it, do some cleanup to related code and documentation