Create and initialize udptl only when dialog negotiates for image media

Prior to this patch, the udptl struct was allocated and initialized when a
dialog was associated with a peer that supported T.38, when a new SIP
channel was allocated, or what an INVITE request was received.  This resulted
in any dialog associated with a peer that supported T.38 having udptl support
assigned to it, including the UDP ports needed for communication.  This
occurred even in non-INVITE dialogs that would never send image media.

This patch creates and initializes the udptl structure only when the SDP
for a dialog specifies that image media is supported, or when Asterisk
indicates through the appropriate control frame that a dialog is to support
T.38.

(closes issue ASTERISK-16698)
Reported by: under
Tested by: Stefan Schmidt
Patches: udptl_20120113.diff uploaded by mjordan (License #6283)

(closes issue ASTERISK-16794)
Reported by: Elazar Broad
Tested by: Stefan Schmidt

review: https://reviewboard.asterisk.org/r/1668/
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Merged revisions 351027 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 351028 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@351029 65c4cc65-6c06-0410-ace0-fbb531ad65f3