pgsql: Don't set PAM_RHOST for Unix sockets.

Don't set PAM_RHOST for Unix sockets.

Since commit 2f1d2b7a we have set PAM_RHOST to "[local]" for Unix
sockets. This caused Linux PAM's libaudit integration to make DNS
requests for that name. It's not exactly clear what value PAM_RHOST
should have in that case, but it seems clear that we shouldn't set it
to an unresolvable name, so don't do that.

Back-patch to 9.6. Bug #15520.

Author: Thomas Munro
Reviewed-by: Peter Eisentraut
Reported-by: Albert Schabhuetl
Discussion: https://postgr.es/m/15520-4c266f986998e1c5%40postgresql.org

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/63d8350665c820203fd48e5690af01c33801fe46

Modified Files
--------------
src/backend/libpq/auth.c | 50 +++++++++++++++++++++++++++++-------------------
1 file changed, 30 insertions(+), 20 deletions(-)