journal: Move more pattern matching logic into pcre2-util

journal: Move more pattern matching logic into pcre2-util

To avoid having "#if HAVE_PCRE2" all throughout the code, let's
confine the pcre2 header specific stuff to pcre2-util.c. Instead of
exposing all the individual symbols from pcre2, let's only expose
three high level functions that do all we need:

- pcre2_pattern_compile(): Compile the regex
- pcre2_pattern_matches(): Check if the compiled regex matches a message
- pcre2_pattern_free(): Free the compiled regex

We expose the compiled pcre2 pattern (which is of type pcre2_code *) as
a void pointer to avoid having to include pcre2.h in all code where we
work with compiled pcre2 patterns. For readability, we typedef void
to pcre2_pattern and use that as the type specifier for compiled pcre2
patterns.