[RFD]: deprecate the "fg" command

[Michael Snyder]

Does anybody use the "fg" command (a synonym for continue)?
It's been in there since 1991, but never documented.  I'd like to
deprecate it to reduce clutter and the hazard of dangerous typos.
If you tried to say "f" (the frame command) and your finger hit
two keys by accident, the result would be an unintentional continue,
which is generally a bad thing.