Undo revision 1.7: always mangle a #! line containing "python" to point

Undo revision 1.7: always mangle a #! line containing "python" to point
to the current Python interpreter (ie. the one used for
building/installation), even (especially!) if "/usr/bin/env" appears in
the #! line.

Rationale: installing scripts with "#!/usr/bin/env python" is asking for
trouble, because
  1) it might pick the wrong interpreter (not the one used to
     build/install the script)
  2) it doesn't work on all platforms (try it on IRIX 5, or on Linux
     with command-line options for python)
  3) "env" might not be in /usr/bin