Make plpythonu language use plpython2 shared library directly.

Make plpythonu language use plpython2 shared library directly.

The original scheme for this was to symlink plpython.$DLSUFFIX to
plpython2.$DLSUFFIX, but that doesn't work on Windows, and only
accidentally failed to fail because of the way that CREATE LANGUAGE created
or didn't create new C functions.  My changes of yesterday exposed the
weakness of that approach.  To fix, get rid of the symlink and make
pg_pltemplate show what's really going on.