chan_dahdi: Add dialmode option for FXS lines.

Currently, both pulse and tone dialing are always enabled
on all FXS lines, with no way of disabling one or the other.

In some circumstances, it is desirable or necessary to
disable one of these, and this behavior can be problematic.

A new "dialmode" option is added which allows setting the
methods to support on a per channel basis for FXS (FXO
signalled lines). The four options are "both", "pulse",
"dtmf"/"tone", and "none".

Additionally, integration with the CHANNEL function is
added so that this setting can be updated for a channel
during a call.

Resolves: #35
ASTERISK-29992

UserNote: A "dialmode" option has been added which allows
specifying, on a per-channel basis, what methods of
subscriber dialing (pulse and/or tone) are permitted.

Additionally, this can be changed on a channel
at any point during a call using the CHANNEL
function.

(cherry picked from commit 82d7bb49dd)

configs/samples/chan_dahdi.conf.sample

@@ -1131,10 +1131,19 @@ pickupgroup=1
 ;
 ; For FXO (FXS signalled) devices, whether to use pulse dial instead of DTMF
 ; Pulse digits from phones (FXS devices, FXO signalling) are always
-; detected.
+; detected, unless the dialmode setting has been changed from the default.
 ;
 ;pulsedial=yes
 ;
+; For FXS (FXO signalled) devices, the dialing modes to support for the channel.
+; By default, both pulse and tone (DTMF) dialing are always detected.
+; May be set to "pulse" if you only want to allow pulse dialing on a line.
+; May be set to "dtmf" or "tone" to only allow tone dialing on a line.
+; May be set to "none" to prevent dialing entirely.
+; You can also change this during a call using the CHANNEL function in the dialplan.
+;
+;dialmode=both
+;
 ; For fax detection, uncomment one of the following lines.  The default is *OFF*
 ;
 ;faxdetect=both