SIP support for keep-alive event

SIP support for keep-alive event

keep-alive events are used by Sipura/Linksys for NAT keepalive.
There currently don't appear to be any problems with NAT, but
everytime a keep-alive event is received, Asterisk responds with a
"489 Bad event".  This error may indicate to a user that NAT
problems exist just because this even is not supported.  Now,
rather than respond with an error, the packet is consumed and
a "200 ok" is sent just to indicate we received the packet.

(issue #15084)
Patches:
      chan_sip.keepalive.v1.diff uploaded by IgorG (license 20)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@215466 65c4cc65-6c06-0410-ace0-fbb531ad65f3