Fix possible buffer overrun in hba.c.

Fix possible buffer overrun in hba.c.

Coverty reports a possible buffer overrun in the code that populates the
pg_hba_file_rules view.  It may not be a live bug due to restrictions
on options that can be used together, but let's increase MAX_HBA_OPTIONS
and correct a nearby misleading comment.

Back-patch to 10 where this code arrived.

Reported-by: Julian Hsiao
Discussion: https://postgr.es/m/CADnGQpzbkWdKS2YHNifwAvX5VEsJ5gW49U4o-7UL5pzyTv4vTg%40mail.gmail.com