PCRE2 has way better performance than the OpenBSD
library (something around 20 times faster).

The following flags are enabled for every pattern:

- PCRE2_UTF
- PCRE2_MATCH_INVALID_UTF
- PCRE2_NO_UTF_CHECK

All the others are optional.

Changes made:

- Adds PCRE2 as subproject.
- Changes the API away from POSIX to PCRE2.
- Edits many regex patterns because:
 - ' ' is skipped in patterns, if the EXTENDED flag is set for matching. '\s' must be set now.
 - '.' doesn't match newlines by default.
- Changes the API so matches and their groups are bundled into PVectors.
- Moves the regex component to rz_util.

Meson currently doesn't support subprojects to be native and non-native at the same time.
See: mesonbuild/meson#10947
Unfortunately, sdb depends on rz_util which in turn depends on PCRE2.
Excluding PCRE2 from the native build makes linking of rz_util not possible anymore.
Adding it, will make Meson complain that the dependencies cannot be mixed.

Hence, we compile a copy of PCRE2 for the native build if required.