It's safe to terminate the proxy at any point in its lifecycle, so
there's no point using signalfd() (and the associated select() +
non-blocking I/O gubbins) in it. We might want to use non-blocking
I/O in the future for other reasons, of course, at which point it
might become sensible to use signalfd() again. For now, this makes
us reliably responsive to TERM,INT and QUIT in a way that we weren't
previously.
