Fix some more fax test errors due to needing the peer in a bridge

In r389799, a number of fax errors in gateway mode were fixed by using the
appropriate function to get a channel's peer while in a bridge. This patch
does two things:
(1) It uses the same function in res_fax_spandsp while starting the fax
    gateway. Without this, the fax gateway will not actually start up, as
    res_fax_spandsp also must inspect the channel's peer in a two-party
    bridge
(2) It refactors some ao2 objects in sendfax_exec to use RAII_VAR. This was
    reverted in r389799 as some off nominal paths were getting hit without
    the fix in (1) that indicated an ao2 object issue; this turned out to
    be a red herring (which is an odd phrase)


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@389827 f38db490-d61c-443f-a65b-d21fe96a405b