Allow custom search filters to be configured for LDAP auth

Allow custom search filters to be configured for LDAP auth

Before, only filters of the form "(<ldapsearchattribute>=<user>)"
could be used to search an LDAP server.  Introduce ldapsearchfilter
so that more general filters can be configured using patterns, like
"(|(uid=$username)(mail=$username))" and "(&(uid=$username)
(objectClass=posixAccount))".  Also allow search filters to be included
in an LDAP URL.

Author: Thomas Munro
Reviewed-By: Peter Eisentraut, Mark Cave-Ayland, Magnus Hagander
Discussion: https://postgr.es/m/CAEepm=0XTkYvMci0WRubZcf_1am8=gP=7oJErpsUfRYcKF2gwg@mail.gmail.com