Add the possibility to delete all the contexts registered by a certain...

Add the possibility to delete all the contexts registered by a certain registrar with ast_merge_and_delete routine; make it the default behaviour when reloding extensions


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@1187 65c4cc65-6c06-0410-ace0-fbb531ad65f3