Discussion of this subject has yielded that it is not actually acceptable to change
T.38 parameters after the initial reinvite but declining is harsh and can cause the
fax to fail when it may be possible to allow it to continue. This patch changes things
so that additional T.38 reinvites are accepted but parameter changes ignored. This gives
the fax a fighting chance.

(closes issue #15610)
Reported by: huangtx2009


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@210817 65c4cc65-6c06-0410-ace0-fbb531ad65f3