s3:utils: add new 'net ads setspn add' subcommand

s3:utils: add new 'net ads setspn add' subcommand

This patch adds 'add' to the 'net ads setspn' subcommand

(see https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-R2-and-2012/cc731241(v=ws.11)

Usage:

     net ads setspn add <computer> <SPN>

Note: <computer> is optional, if not specified the computer account
associated with value returned by lp_netbios_name() is used instead.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>