Master was disconnecting its workers as soon as they both started up.
Meanwhile, the workers were trying to listen. Its a race, sometimes the
disconnect would happen between when worker gets the response message,
and acks that message with a 'listening'. This worked OK after v0.11
introduced a behaviour where disconnect would always exit the worker,
but once that backwards-incompatible behaviour is removed, the worker
lives long enough to try and respond to the master, and child_process
errors at the attempt to send from a disconnected child.

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.

process.exit() tends to hide bugs, both in tests and node.js.
Rewrite the tests so that the event loop exits naturally.

Implement support for debugging cluster workers. Each worker process
is assigned a new debug port in an increasing sequence.

I.e. when master process uses port 5858, then worker 1 uses port 5859,
worker 2 uses port 5860, and so on.

Introduce new command-line parameter '--debug-port=' which sets debug_port
but does not start debugger. This option works for all node processes, it
is not specific to cluster workers.

Fixes joyent/node#5318.