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.