Add extra sanity checking to the in-place update routine. Sometimes you

Add extra sanity checking to the in-place update routine. Sometimes you
find two users with the same UID (i.e. root and toor), but yp_mkdb(8)
forbits duplicate keys, so only one of them will end up in the *.byuid
maps (probably toor, since it comes after root in the template file).
If I asked rpc.yppasswdd(8) to change toor's password, it would update
the *.byname maps correctly, but incorrectly modify root's entry in
the *.byuid maps since the only matching record with UID=0 in those
maps belongs to root.

To fix this, we check that both the name and UID are correct before trying
to write new entries to the maps.