Replace popen/setjmp/pclose with a manual pipe/fork/exec/wait.

Replace popen/setjmp/pclose with a manual pipe/fork/exec/wait.

We can limit the time we wait on wall(1) without the complexity
inherent to setjmp.

Actually wait (instead of waitpid) to pick up any straggler wall
processes from prior timewarn() calls.

With a tweak from millert@ to ensure we don't accidentally close
stdin before we exec wall.

ok millert@ tb@