support for LDAP URLs

Here is a patch to support RFC 2255 LDAP URLs in pg_hba.conf. So,
instead of, say

host ... ldap ldapserver=ldap.example.net ldapbasedn="dc=example, dc=net" ldapsearchattribute=uid

you could write

host ... ldap lapurl="ldap://ldap.example.net/dc=example,dc=net?uid?sub"

Apache and probably other software uses the same format, and it's easier
to have a common format for all such configuration instead of having to
translate the information provided by the LDAP admin into each
software's particular configuration spellings.

I'm using the OpenLDAP-provided URL parsing routine, which means this
wouldn't be supported on Windows. But we already support different
authentication settings on different platforms, so this didn't seem such
a big problem.