chan_dahdi: Never send MWI while off-hook.

chan_dahdi: Never send MWI while off-hook.

In some circumstances, it is possible for the do_monitor thread to
erroneously think that a line is on-hook and send an MWI FSK spill
to it when the line is really off-hook and no MWI should be sent.
Commit 0a8b3d34673277b70be6b0e8ac50191b1f3c72c6 previously fixed this
issue in a more readily encountered scenario, but it has still been
possible for MWI to be sent when it shouldn't be. To robustly fix
this issue, query DAHDI for the hook status to ensure we don't send
MWI on a line that is actually still off hook.

Resolves: #928