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.

For more comments, read the patches@python.org archives.
For documentation read the comments in mymalloc.h and objimpl.h.

(This is not exactly what Vladimir posted to the patches list; I've
made a few changes, and Vladimir sent me a fix in private email for a
problem that only occurs in debug mode.  I'm also holding back on his
change to main.c, which seems unnecessary to me.)

symlink to a symlink can work.
(Jack)

- Don't print error messages when Py_FrozenFlag is set.

executable files, and directories.  When expecting a module, we also
look for the .pyc or .pyo file.

### If you still have sharedmodules in your Modules/Setup file, remove it! ###

searches /usr/local, /usr and /.

would always be a hit.   Prevent this by only using VPATH if we know
we are in the build directory.

string_append().  These must be artifacts of GvR's rewrite.

Fixed some typos in the leading comment (and re-filled the
paragraphs).

Hope you don't mind, Guido.

Idea and first three implementation rounds due to Barry -- after that
I spent another day on it, hopefully it's enough for now :-)

(Wait for the checkin to Setup.in.)