When a error in T.38 negotiation happens or its rejected on a channel the

 state of the channel reverts to unknown this should be rejected.
 
 this is important for negotiating T.38 gateway see #13405

 This patch adds a option T38_REJECTED that behaves as T38_DISABLED except it reports state rejected.

 Trivial Change to res_fax to honnor UNAVAILABLE and REJECTED states.

 (closes issue #18889)
 Reported by: irroot
 Tested by: irroot, darkbasic, 	mnicholson

 Review: https://reviewboard.asterisk.org/r/1115



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@319087 65c4cc65-6c06-0410-ace0-fbb531ad65f3