Shell tutorial added

["Perry E. Metzger" <perry%piermont.com@localhost>]

I've added S.R. Bourne's 1970s vintage tutorial on the shell to
src/bin/sh/USD.doc, as very very very extensively updated by me to
reflect the last 30+ years of changes to the shell. Comments on my
changes are actively solicited.

I did not test this with a full system build, though I did add what I
believe to be the correct sets lines. If I broke something, my
apologies.

One messy bit: I committed a file of v7 era "refer" stanzas (Rv7man)
that was originally located in v7 in /usr/dict/papers/ to the shell
docs directory. The original directory has no equivalent in a modern
system. However, if people commit updates of other v7 era documents,
this file probably should go in a common location, not in
share/doc/usd/03.shell -- I just put it there for now because there
aren't any other users yet. Suggestions for a better location are
solicited.

Perry
-- 
Perry E. Metzger                perry%piermont.com@localhost