debugger: make busy loops SIGUSR1-interruptible

Commit 30e5366b ("core: Use a uv_signal for debug listener") changed
SIGUSR1 handling from a signal handler to libuv's uv_signal_*()
functionality to fix a race condition (and possible hang) in the
signal handler.

While a good change in itself, it made it impossible to interrupt
long running scripts.  When a script is stuck in a busy loop, control
never returns to the event loop, which in turn means the signal
callback - and therefore the debugger - is never invoked.

This commit changes SIGUSR1 handling back to a normal signal handler
but one that treads _very_ carefully.