* in py-checker-run, fall back to read-string if read-shell-command
  (XEmacs-specific) is not available.

* highlight variables would mask builtins as if they were keywords.

submitted by Bernard Herzog.  Closes patch 779830.

evil - if the last grubbed buffer didn't happen to be the right one,
you couldn't remedy.

Mainline emacs compat - don't use third arg to buffer-substring (which
was for explicitly identifying the buffer in which to seek the
substring, and which turns out to be unnecessary).

eg "pdb>".

sessions (and some cosmetic wording changes).

introduced when shifting around some code, and added some redundancy
to reduce chances of hitting the wrong source code.  (This is
experimental - it will improve the accuracy, but will reduce the
ability of the user to deliberately select the buffer they want the
buffer grubbing stuff to find.  I think the accuracy improvement will
be worth it, but am not sure, so may remove this.)

the reported path.  (Eg, precompiled scripts with a file path suitable
for a different host, scripts actually running on a remote system or
with no valid path, like Zope through-the-web python scripts.)

On failing to find the code on the reported path, pdbtrack takes the
function name and looks through the buffers, from most to least
recent, seeking the first python-mode buffer that either is named for
the function or has a definition (def or class) for that function.  So
to get source tracking for code that's not located where the path
indicates, you put a copy of the script in a buffer, and pdbtrack will
find it.

Also, fixed a small bug so pdbtrack now properly presents the overlay
arrow when you run the pdb 'w'here command.