sig_analog: Add Called Subscriber Held capability.

sig_analog: Add Called Subscriber Held capability.

This adds support for Called Subscriber Held for FXS
lines, which allows users to go on hook when receiving
a call and resume the call later from another phone on
the same line, without disconnecting the call. This is
a convenience mechanism that most real PSTN telephone
switches support.

ASTERISK-30372 #close

Resolves: #240

UserNote: Called Subscriber Held is now supported for analog
FXS channels, using the calledsubscriberheld option. This allows
a station  user to go on hook when receiving an incoming call
and resume from another phone on the same line by going on hook,
without disconnecting the call.

(cherry picked from commit 52283301e8d4e7e8bf980499bd381b53adcedbe3)