get tcsh to support bash-style command-not-found hooks

[geofft]

This should be a reasonably easy patch to sh.exec.c:pexerr() or so.

Surprisingly, I can't seem to find evidence of an existing patch to do this. It's probably worth looking a little harder and seeing if upstream (or any of the distros) have attempted or otherwise considered this before we write our own patch and send it upstream to tcsh or the distros.