Add alw as a valid file extension for alaw and ulw as a valid file extension for ulaw.

(closes issue #14001)
Reported by: henrikw
Patches:
      alw.diff uploaded by henrikw (license 627)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@161869 65c4cc65-6c06-0410-ace0-fbb531ad65f3