s3: shortcut gid_to_sid when "ldapsam:trusted = yes"

s3: shortcut gid_to_sid when "ldapsam:trusted = yes"

The normal gid_to_sid behaviour is to call sys_getgrgid()
to get the name for the given gid and then call the
getsamgrnam passdb method for the resulting name.

In the ldapsam:trusted case we can reduce the gid_to_sid
operation to one simple search for the gidNumber attribute
and only get the sambaSID attribute from the correspoinding
LDAP object. This reduces the number of ldap roundtrips
for this operation.

metze
(similar to commit 0fb99386d41241f62312d4bb535976344e5d6492)