sorcery, bucket: Change observer remove calls to take const callbacks struct.

* Make ast_sorcery_observer_remove() accept a const callbacks struct.

* Make ast_sorcery_observer_remove() tolerant of the sorcery parameter
being NULL.  Now it can be called within a module unload routine if the
sorcery initialization fails.

* Fix ast_sorcery_observer_add() to fail if the container link fails.
........

Merged revisions 403324 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@403327 f38db490-d61c-443f-a65b-d21fe96a405b
res_pjsip_session: Add support for PJMEDIA_SDP_NEG_ALLOW_MEDIA_CHANGE flag.

Newer versions of PJSIP have changed to using a flag for the
PJMEDIA_SDP_NEG_ALLOW_MEDIA_CHANGE instead of a define. This adds a
configure check to detect the presence of the flag and use it if found.
........

Merged revisions 403329 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@403330 f38db490-d61c-443f-a65b-d21fe96a405b