Fix some remaining T.38 negotiation problems in app_fax.

Revision 205696 did not quite fix all the issues with the T.38 negotiation
changes and app_fax; this patch corrects them, along with a couple of other
minor issues.

(closes issue #15480)
Reported by: dimas
Patches:
      test2-15480.patch uploaded by dimas (license 88)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@205770 65c4cc65-6c06-0410-ace0-fbb531ad65f3