!pref: Add the "new" sms modes to usage text.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
!pref, !sms, merlin.cfg: Better checks for whether an sms service is configured before allowing it to be set in !pref. Better error messages for bad smsmodes in !sms.

Signed-off-by: Martin Stone <martin@d7415.co.uk>