s3:winbindd: use struct unixid instead of uint64 in Sids2Xids parent<->child

s3:winbindd: use struct unixid instead of uint64 in Sids2Xids parent<->child

This implicitly also hands the type of the resulting unix-id that the idmap
backend has created back to the caller. This is important for backends that
would set a broader type than the requested one, e.g. rid backend returning
BOTH instead of UID or GID.

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>