Show stack trace but not quit on SIGQUIT

[xiaq]

Right now SIGQUIT will make Elvish show a stack trace and crash. This is intended, since that agrees with the intended semantics of SIGQUIT.

However, most other shells just ignore SIGQUIT, so the fact that Ctrl-\ kills Elvish can be a bit surprising.

It is sometimes useful, for debugging, to get a stack trace when Elvish is running. So let's make SIGQUIT just show a stack trace but not crash Elvish.

Admittedly, the most appropriate signal for this is SIGINFO, but this signal is not universally available -- in fact, only available on FreeBSD, AFAIK.