Security fixes for smart terminals

[utzoo!decvax!ucbvax!ihnss!houxi!npois!harpo!cbosg!teklabs!tekmdp!grahamr]

	The problem is sending ARBITRARY data upon request FROM the
system.  It doesn't include sending the terminal type--if it's in rom
or given at the keyboard--or sending the cursor position.  Several
fixes come to mind, from a switch that turns off these features to a
keyboard- or rom-defined prefix for such transmissions.  It's clear
that the problem is in the terminal.  Any software solutions are
probably full of holes.  Anybody have a PROM scrambler?
	"mesg n" prevents opening, not writing.  All that's needed is
to complete the open call before "mesg" runs.  Letter bombs are also a
problem.  My terminal has a keyboard lock feature.  It's easy to send a
letter that locks my keyboard while it does its dirty work.  I think
there's a "reset" button I can hit, but I probably won't hit it quick
enough.  Besides, it can be reprogrammed!
	A kludge for MH systems to get around the letter bomb problem
is to have "l" rewritten as something like:
		cat $* | sed -n l
This might be done on a per-user basis if show used execvp.  It doesn't.