Update CONTROL STREAM FILE to accept an 'offsetms' parameter

This patch allows starting playback of audio through the CONTROL STREAM FILE
AGI command to start at a particular offset. It will also return the final
position of the file in the 'endpos' attribute.

(closes issue ASTERISK-17803)
Reported by: Murray Melvin
patches:
  res_agi.c.r316293.diff uploaded by murraytm (license 6221)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@395906 65c4cc65-6c06-0410-ace0-fbb531ad65f3