slapi-nis should allow password update on a virtual entry
During password modification ext. op (1.3.6.1.4.1.4203.1.11.1),
if the target entry is in the compat tree, slapi-nis should
remap the entry to the real entry.
This needs to be done in a pre-op extop that calls the callback
function handling a given OID.
The password mod. callback does a reverse mapping of
extop USERID and set it in SLAPI_TARGET_SDN.
https://fedorahosted.org/freeipa/ticket/5955