Support FXS module Polarity Reversal on remote party Answer and Hangup

FXS lines normally connect to a telephone. However, when FXS lines are routed
to an external PBX or Key System to act as "external" or "CO" lines, it is
extremely difficult, if not impossible for the external PBX to know when
the call has been disconnected without receiving a polarity reversal on the line.

Now using answeronpolarityswitch and hanguponpolarityswitch keywords that
previously were used only for FXO ports, now applies like functionality for
an FXS port, but from the connected equipment's point of view.

(closes issue #17318)
Reported by: armeniki
Patches: 
      fxs_linepolarity.diff5.txt uploaded by alecdavis (license 585)
Tested by: alecdavis

Review: https://reviewboard.asterisk.org/r/797/



git-svn-id: http://svn.digium.com/svn/asterisk/trunk@278809 f38db490-d61c-443f-a65b-d21fe96a405b

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