res_fax: Add T.38 negotiation timeout option

This change makes the T.38 negotiation timeout configurable via
't38timeout' in res_fax.conf or FAXOPT(t38timeout). It was previously
hard coded to be 5000 milliseconds.

This change also handles T.38 switch failures by aborting the fax since
in the case where this can happen, both sides have agreed to switch to
T.38 and Asterisk is unable to do so.

Review: https://reviewboard.asterisk.org/r/4320/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@430415 65c4cc65-6c06-0410-ace0-fbb531ad65f3