netcmd: Add sanity-check for invalid domain rename args

netcmd: Add sanity-check for invalid domain rename args

We are suggesting to users that it's safe to run a renamed domain in
parallel with the old backed-up domain. However, this would not be the
case if the user (foolishly) "renames" their domain using the exact same
NetBIOS name or DNS realm.

Using the same DNS realm fails later on (updating the dnsRoot values),
but using the same NetBIOS name actually succeeds. While we can't make
samba tools completely idiot-proof, we can protect users from the most
basic of (potentially unintended) errors with some simple sanity-checks.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>