Extract a useful routine from the softmix bridge technology.

* Extract a useful routine from the softmix bridge technology for other
technologies.  Make other technologies use it if they can.

* Made native and 1-1 bridges write to all parties if the bridge channel
writing the frame into the bridge is NULL.  Softmix will also do the same
for frame types that make sense.

* Tweak the bridge write routine return value meaning and adjust the
bridge technologies to match.


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@392514 f38db490-d61c-443f-a65b-d21fe96a405b