idmap_nss: Add a parameter to use UPNs instead of plain names

idmap_nss: Add a parameter to use UPNs instead of plain names

idmap config <DOMAIN> : backend = nss
idmap config <DOMAIN> : use_upn = yes|no

When translating a Unix ID to a SID the module calls get[pwu|grg]id() but the
name returned by some NSS modules might be a UPN instead of a plain name. If
the new parameter is enabled the returned name will be parsed and correctly
handled.

On the other hand, when translating a SID to a Unix ID the module first
resolves the SID to a domain + name, and then calls get[pw|gr]name() with the
plain name, or the UPN if the new parameter is enabled.

Signed-off-by: Samuel Cabrero <scabrero@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>