BUG #15520: PAM authentication + domain socket -> DNS query for symbolic hostname [local]

The following bug has been logged on the website:

Bug reference:      15520
Logged by:          Albert Schabhuetl
Email address:      albert_schabhuetl@yahoo.de
PostgreSQL version: 10.4
Operating system:   RHEL 7.4
Description:        

When PAM authentication is configured for local connections via domain
socket,
during authentication the backend process sends out a DNS query 
for the following symbolic hostname: [local]

This is unnecessary as it is not intended that this hostname will ever
resolve to a valid IP address,
and depending on how DNS queries are answered in the actual system it can
cause authentication delays.

Theory why this happens:

Since this commit
https://github.com/postgres/postgres/commit/2f1d2b7a75fecad25295cb3f453503eb6a176d4f#diff-f5a9c53142c3595fe47ebf5146457c6b
during PAM authenticaion the functions
	pg_getnameinfo_all
	pam_set_item
are called, in order to support the PAM configuration option
"pam_use_hostname".

In the case of a TCP connection and pam_use_hostname = 0,
	pg_getnameinfo_all still calls the library function
	getnameinfo
but with flags = 0 to achieve "do nothing" behaviour.

In the case of a domain socket connection,
regardless of pam_use_hostname,
	pg_getnameinfo_all calls 
	getnameinfo_unix,
which ignores the flags parameter and invariably
sets the hostname to a symbolic text - [local] in our case.

When subsequently the PAM library function 
	pam_set_item
is called, it gets the symbolic hostname [local]
and probably it is this function which sends the DNS request for this
hostname.