Still with plenty of repeated code to be factored out.

The implementation is not trivial, and I'm still not entirely clear
on the use case for Varnish/VCL. We probably will need partial
matching to implement matches against the request body.

so that backref/namedref are guaranteed to return strings in workspace.

There's still a bit of code repetition to be factored out.

for use by the backref/namedref functions.

This brings about a major refactoring, to factor out common code for
regex compiles and matches.

failure messages.

after using pcre2_match_data_create_from_pattern() to create match_data.

This requires a test in configure.ac for whether UTF8 is enabled in
the PCRE2 lib against which the VMOD is built.

and recursion_limit, and test compile option use_offset_limit

note that I don't know at the moment how to test never_backslash_c