Fix T.38 support when used with chan_local in between.

Users of the T.38 API can indicate AST_T38_REQUEST_PARMS on a channel to request that the
channel indicate a T.38 negotiation with the parameters present on the channel. The return
value of this indication is expected to be AST_T38_REQUEST_PARMS upon success but with
chan_local involved this could never occur.

This fix changes chan_local to always return AST_T38_REQUEST_PARMS for this situation. If
the underlying channel technology on the other side does not support T.38 this would have
been determined ahead of time using ast_channel_get_t38_state and an indication would
not occur.

(closes issue ASTERISK-20229)
Reported by: wdoekes
Patches:
     ASTERISK-20229.patch uploaded by wdoekes (license 5674)

Review: https://reviewboard.asterisk.org/r/2070/
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Merged revisions 373705 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373706 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 373707 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373708 65c4cc65-6c06-0410-ace0-fbb531ad65f3