Fix station ringback; trunk hangup issues in SLA

This patch fixes two bugs:
 * If an outbound call is made from a SLA phone using SLAStation, then there is
   no ringtone audible to the phone that originates the call. The indication of
   the ringing was not being passed to the SLA station; this patch fixes that
   by passing through the progress indications.
 * If an SLA station hangs up before the called party answers, then the channel
   to the called party continues to ring until a timeout occurs. If the called
   party manages to answer, Asterisk attempts to connect the called party to
   a non-existant MeetMe room. This patch corrects the behavior by abandoning
   the call attempt if it detects that the SLA station is no longer in use
   while attempting to call the called party.

Review: https://reviewboard.asterisk.org/r/2275/

(closes issue ASTERISK-20462)
Reported by: dkerr
patches:
  asterisk-11-bugid20440+20462.patch uploaded by dkerr (license 5558)
  asterisk-11-bugid20462.patch uploaded by dkerr (license 5558)

(closes issue ASTERISK-20440)
Reported by: dkerr
patches:
  asterisk-11-bugid20440.patch uploaded by dkerr (license 5558)
  asterisk-11-bugid20440+20462.patch uploaded by dkerr (license 5558)
........

Merged revisions 379825 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@379826 f38db490-d61c-443f-a65b-d21fe96a405b