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

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

The normal uid_to_sid behaviour is to call sys_getpwuid()
to get the name for the given uid and then call the
getsampwnam passdb method for the resulting name.

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

Michael