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

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

This patch adds basic functionality not unlike the setspn.exe
command that is provided by windows for adminsistering SPN on
the AD. (see https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-R2-and-2012/cc731241(v=ws.11)

Only the basic list operation (that corresponds to the -l
    switch for setspn.exe is implemented)

Usage:

     net ads setspn list <computer>

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>