chan_dahdi: Allow autoreoriginating after hangup.

Currently, if an FXS channel is still off hook when
all calls on the line have hung up, the user is provided
reorder tone until going back on hook again.

In addition to not reflecting what most commercial switches
actually do, it's very common for switches to automatically
reoriginate for the user so that dial tone is provided without
the user having to depress and release the hookswitch manually.
This can increase convenience for users.

This behavior is now supported for kewlstart FXS channels.
It's supported only for kewlstart (FXOKS) mainly because the
behavior doesn't make any sense for ground start channels,
and loop start signalling doesn't provide the necessary DAHDI
event that makes this easy to implement. Likely almost everyone
is using FXOKS over FXOLS anyways since FXOLS is pretty useless
these days.

ASTERISK-30357 #close

Resolves: #224

UserNote: The autoreoriginate setting now allows for kewlstart FXS
channels to automatically reoriginate and provide dial tone to the
user again after all calls on the line have cleared. This saves users
from having to manually hang up and pick up the receiver again before
making another call.

configs/samples/chan_dahdi.conf.sample

@@ -1097,6 +1097,15 @@ pickupgroup=1
 ;                        polarity switch and hangup polarity switch.
 ;                        (default: 600ms)
 ;
+; For kewlstart FXS (FXO signalled) ports only:
+; When all calls towards a DAHDI channel have cleared, automatically
+; reoriginate and provide dial tone to the user again, so s/he can
+; make another call without having to cycle the hookswitch manually.
+; This only works for kewlstart (fxo_ks) lines!
+; Dial tone will be provided only after the loop disconnect has finished.
+;
+;autoreoriginate=yes
+;
 ; On trunk interfaces (FXS) it can be useful to attempt to follow the progress
 ; of a call through RINGING, BUSY, and ANSWERING.   If turned on, call
 ; progress attempts to determine answer, busy, and ringing on phone lines.