If multiple [general] contexts occur from sip.conf (usually due to external includes), merge them.

The original implementation of this did the merging of all contexts with the
same name in the realtime layer, but that implementation severely breaks
drivers which use the same context name (e.g. iax.conf, type={peer,user}).
Therefore, the implementation needs to do the merging for particular entries
only, based upon what contexts would allow that in the channel driver itself.
This implementation is for chan_sip only, but others could be added in the
future.

(closes issue #17957)
 Reported by: marcelloceschia
 Patches: 
       chan-sip_parsing-general_branch162.patch uploaded by marcelloceschia (license 1079)
 Tested by: tilghman


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@316428 65c4cc65-6c06-0410-ace0-fbb531ad65f3