E3-1066: Refactor require.c

General refactoring of the code in require.c. Includes:

• Removing checks for installation methods that we do not use
• Removes versionstr (which we have never used)
• Removes ifexists: there was an option to do require asyn,ifexists, which we have never supported
• Removes support for passing arguments to require that used to be passed to automatically run startup scripts (again, we never supported that)
• Only looks for files where we actually expect them to be
• Remove foreachLoadedLib - allows one to run a function for each library, but never used
• Cleanup debug to always use macro
• Removed some static firstTime checks by running code in better places
• Drop support for gcc < 3
• And most importantly, actually tried to refactor the code in require_priv. This is the main one that really needs review. @lucasmagalhaes , in particular, I would appreciate your thoughts here.