Conditionally load the AGI command gosub, depending on whether or not res_agi

has been loaded, fix a return value in the loader, and ensure that the help
workhorse header does not print on load.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@120602 65c4cc65-6c06-0410-ace0-fbb531ad65f3