various cleanups:

  use API call for finding channel by name prefix
  code formatting to match guidelines (lost about half the of the indenting)
  remove useless automatic variable initializations
  don't set the spying channel's read format to SLINEAR when we don't do anything with the voice frames we read from it anyway
  use proper option argument checking for volume argument



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@38368 65c4cc65-6c06-0410-ace0-fbb531ad65f3