Minor improvements to app_fax.

Minor improvements to app_fax.

This patch makes some small changes to handle watchdog timeouts in a better way,
and also uses a 'cleaner' method of including the spandsp header files.

(closes issue #14769)
Reported by: andrew
Patches:
      app_fax-20090406.diff uploaded by andrew (license 240)
      v1-14769.patch uploaded by dimas (license 88)
Tested by: freh, deti, caspy, dimas, sgimeno, Dovid

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@210777 65c4cc65-6c06-0410-ace0-fbb531ad65f3