Make "executable Python script" a shell wrapper which calls Python source file

- Avoid problem with #! line length limit, since #! no longer calls wrapper
- Avoid problems with Python documentation and __future__ imports, since
  Python source file is not modified (except by possibly adding #! line)
- Rename $(pybin_SCRIPTS) to $(pybin_scripts), since installation of Python
  scripts is now handled manually, not by Automake
- If installing into the same exec_prefix as Python, install the Python source
  script with a #!$(PYTHON) line; otherwise, install a modified shell wrapper
  script in $(bindir) and Python source script in $(pkglibexecdir)