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

Signed-off-by: Martin Stone <martin@d7415.co.uk>
(cherry picked from commit f58b2a0a69ae6fb40cd25f08a9621df1b667e77d)
!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>
(cherry picked from commit d2343498038b68e47b7fbbaec4637bc4d569b39b)