libpq: Fix inadvertent change in .pgpass lookup behavior.

Commit 274bb2b3 caused password file
lookups to use the hostaddr in preference to the host, but that was
not intended and the documented behavior is the opposite.

Report and patch by Kyotaro Horiguchi.

Discussion: http://postgr.es/m/20170428.165432.60857995.horiguchi.kyotaro@lab.ntt.co.jp