using pcomplete for comint.el/shell.el

[Colin Walters]

John Weigley has been having trouble with getting papers signed for his
general Emacs contributions, so he asked me to finish the last bit of
work needed to get shell.el to use pcomplete.el.  

One of the design goals was to avoid loading pcomplete unless the user
actually used the completion facilities.  So this made things a little
bit tricky.  I've attached the patch to shell.el to implement this. 
Other modes will have to do something very similar.  

Or would it be better if we just unconditionally required pcomplete.el
for shell.el?

After this is implemented, we can just remove a lot of cruft in
comint.el.  

Also, one issue to consider is that this will break backwards
compatibility with modes deriving from comint.

shell.diff
Description: Text document