pgsql: Replace hardcoded mentions of pg_hosts.conf with GUC

Replace hardcoded mentions of pg_hosts.conf with GUC

Three error messages were using the default file name pg_hosts.conf
and not the variable backing the GUC, which would make logging be
confusing for users who have renamed the file using the GUC. Fix
by consistently using the HostsFileName variable.

Backpatch down to v19 where serverside SNI was introduced.

Author: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Reviewed-by: Surya Poondla <suryapoondla4(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/CAN4CZFMARYjQfgyRaCKOXDO=Q91kuKn=pSC02DAOOr23ojhEGQ@mail.gmail.com
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/f74a45a5416fb18c391b3a1651e29ada78106c43

Modified Files
--------------
src/backend/libpq/be-secure-openssl.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)