Current signal handling functionality in the runtime handles all signals, preventing
default behavior for SIGTSTP, SIGTTIN, and SIGTTOU. One proximate effect of this is
that Ctrl-Z from a unix shell does not suspend any Go process that handles signals.
It might be generally convenient to have a way to catch only a specific
subset of all signals, and to have some set of signals have their
default behaviour.
It might also occasionally be convenient to have a way to catch a
signal and then to invoke the signal's default behaviour, or to deliver another signal
to itself or another process.
