Add the tame "exec" request. This allows processes which request

Add the tame "exec" request.  This allows processes which request
"exec" to call execve(2), potentially fork(2) beforehands if they
asked for "proc".  Calling execve is what "shells" (ksh, tmux, etc)
have as their primary purpose.  But meantime, if such a shell has a
nasty bug, we want to mitigate the process from opening a socket or
calling 100+ other system calls.  Unfortunately silver bullets are in
short supply, so if our goal is to stay in a POSIX-y environment, we
have to let shells call execve().  POSIX ate the world, so choices do
we all have?
Warning for many: silver bullets are even more rare in other OS
ecosystems, so please accept this as a narrow lowering of the bar in a
very raised environment.
Commited from a machine running tame "proc exec" ksh, make, etc.