main/rtp_engine: Format NTP timestamps as unsigned ints

main/rtp_engine: Format NTP timestamps as unsigned ints

On some systems, a timeval's tv_sec/tv_usec will be unsigned lont ints, as
opposed to long ints. When the RTP engine formats these as strings, it was
previously formatting them as signed integers, which can result in some
odd negative timestamp values (particularly on 32-bit systems). This patch
formats the values as unsigned long integers.
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Merged revisions 422766 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 422767 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@422768 65c4cc65-6c06-0410-ace0-fbb531ad65f3