frame.c: Make debugging easier

 * ast_frame_subclass2str() and ast_frame_type2str() now return
   a pointer to the buffer that was passed in instead of void.
   This makes it easier to use these functions inline in
   printf-style debugging statements.

 * Added many missing control frame entries in
   ast_frame_subclass2str.

Change-Id: Ifd0d6578e758cd644c96d17a5383ff2128c572fc