Backport of 1.47:

Backport of 1.47:
Fix for #932977: MacOSX does not pass the whole pathname in argv[0] for
#!-scripts, only the filename part, and this can lead to incorrect
initialization of sys.path and sys.executable if there is another python
on $PATH before the one used in #!.

The fix was picked up from the darwinports crowd, thanks!