Towards a new shell subsystem

- Process forms and extended process forms are now
merged.
There are only process forms. Their syntax remains
exactly the same
as before. However, by virtue of this change, it has
become
slightly more liberal. For example (run ls) is now
valid, as is
(run (ls)).

- Process forms have value-level cousins called process
descriptors
with exactly the same composition operators available
for them as
procedures. There's a PROCESS-FORM macro which
converts a process
form into a process descriptor, and you can take
things from there.

This takes care of Tom Lord's criticism at

http://mail.gnu.org/pipermail/guile-devel/2001-August/003291.html