Use a better approach to locating IDLE's default configuration,

allowing it to be run from anywhere, including through a symlink to
the actual idle.py script.

Tools/idle/idle.py

@@ -4,7 +4,7 @@
 import sys
 import IdleConf
 
-idle_dir = os.path.split(sys.argv[0])[0]
+idle_dir = os.path.dirname(IdleConf.__file__)
 IdleConf.load(idle_dir)
 
 # defer importing Pyshell until IdleConf is loaded