Perform the initial renaming of the Bridging API

This patch does the following:
 * It pulls out bridge_channel and puts it into its own translation unit
 * It adds public and protected headers for bridging_channel. Protected
   functions are appropriate only for the Bridging API and sub-classes of a
   bridge.

(issue ASTERISK-22130)


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@395253 f38db490-d61c-443f-a65b-d21fe96a405b
Add missing line terminator to debug message.

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@395254 f38db490-d61c-443f-a65b-d21fe96a405b
Add missing end-of-file line terminators.

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@395255 f38db490-d61c-443f-a65b-d21fe96a405b