Answer video SDP offers properly when videosupport is not enabled.

Copied from Review board:

In issue 12434, the reporter describes a situation in which audio and video 
is offered on the call, but because videosupport is disabled in sip.conf, 
Asterisk gives no response at all to the video offer. According to RFC 3264, 
all media offers should have a corresponding answer. For offers we do not 
intend to actually reply to with meaningful values, we should still reply 
with the port for the media stream set to 0.

In this patch, we take note of what types of media have been offered and 
save the information on the sip_pvt. The SDP in the response will take into 
account whether media was offered. If we are not otherwise going to answer 
a media offer, we will insert an appropriate m= line with the port set to 0.

It is important to note that this patch is pretty much a bandage being 
applied to a broken bone. The patch *only* helps for situations where video 
is offered but videosupport is disabled and when udptl_pt is disabled but 
T.38 is offered. Asterisk is not guaranteed to respond to every media offer. 
Notable cases are when multiple streams of the same type are offered. 
The 2 media stream limit is still present with this patch, too.

In trunk and the 1.6.X branches, things will be a bit different since Asterisk 
also supports text in SDPs as well.

(closes issue #12434)
Reported by: mnnojd

Review: https://reviewboard.asterisk.org/r/311
Review: https://reviewboard.asterisk.org/r/313



git-svn-id: http://svn.digium.com/svn/asterisk/branches/1.4@207423 f38db490-d61c-443f-a65b-d21fe96a405b

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