Refactor more logic for compilation of regular expressions in hba.c

Refactor more logic for compilation of regular expressions in hba.c

It happens that the parts of hba.conf that are planned to be extended
to support regular expressions would finish by using the same error
message as the one used currently for pg_ident.conf when a regular
expression cannot be compiled, as long as the routine centralizing the
logic, regcomp_auth_token(), knows from which file the regexp comes from
and its line location in the so-said file.

This change makes the follow-up patches slightly simpler, and the logic
remains the same.  I suspect that this makes the proposal to add support
for file inclusions in pg_ident.conf and pg_hba.conf slightly simpler,
as well.

Extracted from a larger patch by the same author.  This is similar to
the refactoring done in fc579e1.

Author: Bertrand Drouvot
Discussion: https://postgr.es/m/fff0d7c1-8ad4-76a1-9db3-0ab6ec338bf7@amazon.com