Python on UNIX now trusts PYTHONHOME unconditionally

Modules/getpath.c:

Landmark changed to os.py.

Setting PYTHONHOME now unconditionally sets sys.prefix
(and sys.exec_prefix). No further checks are done whether the
standard lib can be found in that location or not. This is in
sync with the PC subdir getpath implementations.

PC/getpathp.c:

Landmark changed to os.py.

PC/os2vacpp/getpathp.c:

Landmark changed to os.py.

Note: BAW's checkin on exceptions.c eliminates earlier concerns about
a bogus PYTHONHOME value leading to a core dump.  Instead it causes a
useless sys.path and prevents imports.