Some code cleanup in pbx.c

Some code cleanup in pbx.c

* Mostly comment and format changes.

* ast_context_remove_extension_callerid() and ast_add_extension_nolock()
will write lock the found specific context.

* ast_context_find() will now tolerate a NULL name.

* Eliminated some inlined versions of find_context() and
find_context_locked().

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@326985 65c4cc65-6c06-0410-ace0-fbb531ad65f3