core: Use a uv_signal for debug listener

Starting the debugger directly in the SIGUSR1 signal handler results in
a malloc lock contention ~1% of the time.  It hangs the test, which is
annoying on a daily basis to all of us, but it also is pretty terrible
if you actually want to debug a node process that has gone sideways.

Credit to @bnoordhuis for most of this.  I just added the unref which
keeps it from messing up the event loop for other stuff.