LDAP servers can be advertised on a network with RFC 2782 DNS SRV
records.  The OpenLDAP command-line tools automatically try to find
servers that way, if no server name is provided by the user.  Teach
PostgreSQL to do the same using OpenLDAP's support functions, when
building with OpenLDAP.

For now, we assume that HAVE_LDAP_INITIALIZE (an OpenLDAP extension
available since OpenLDAP 2.0 and also present in Apple LDAP) implies
that you also have ldap_domain2hostlist() (which arrived in the same
OpenLDAP version and is also present in Apple LDAP).

Author: Thomas Munro
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/CAEepm=2hAnSfhdsd6vXsM6VZVN0br-FbAZ-O+Swk18S5HkCP=A@mail.gmail.com