sig_analog: Allow immediate fake ring to be suppressed.

sig_analog: Allow immediate fake ring to be suppressed.

When immediate=yes on an FXS channel, sig_analog will
start fake audible ringback that continues until the
channel is answered. Even if it answers immediately,
the ringback is still audible for a brief moment.
This can be disruptive and unwanted behavior.

This adds an option to disable this behavior, though
the default behavior remains unchanged.

ASTERISK-30003 #close
Resolves: #118

UserNote: The immediatering option can now be set to no to suppress
the fake audible ringback provided when immediate=yes on FXS channels.