Ensure ReceiveFax provides a CED tone via T.38

When using res_fax_digium, the T.38 CED tone was not being provided
properly which would cause some incoming faxes to fail. This was not an
issue with res_fax_spandsp since it does not strictly honor the
send_ced flag and sends the CED tone whenever receiving a T.38 fax.

(closes issue FAX-343)
Reported-by: Benjamin Tietz
Patch-by: Kinsey Moore
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Merged revisions 377655 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 377656 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 377657 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@377658 65c4cc65-6c06-0410-ace0-fbb531ad65f3