in pbx_exec is always 1 so it can be removed.

This change also takes away ast_exec_extension(), and lets all
switch functions (exists, canmatch, exec, matchmore) all use the same
prototype, which makes the code a bit cleaner.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@16558 65c4cc65-6c06-0410-ace0-fbb531ad65f3