Changes transport option in sip.conf so that using multiple instances doesn't stack.

Changes transport option in sip.conf so that using multiple instances doesn't stack.

Prior to this patch, Using "transport=" multiple times would cause them to add to one
another like allow/deny. This patch changes that behavior to simply use the transport
option specified last. Also, if no transport option is applied now, the default will
automatically be UDP.

(closes ASTERISK-19352)
Reported by: jamicque
Patches:
	asterisk-19352-transport-warning-message-v1.patch uploaded by Michael L. Young (license 5026)
	issueA19352_no_transport_is_udp.patch uploaded by Walter Doekes (license 5674)
Review: https://reviewboard.asterisk.org/r/1745/diff/#index_header

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@357266 65c4cc65-6c06-0410-ace0-fbb531ad65f3