Handle ExitFailure (-sig) by killing process with signal

On Unix we now use negative exit codes in ExitFailure to indicate that a
process exited due to a signal. This patch implements the case for when
a ExitFailure exception propagates out of the top of main (and is
handled by the topHandler).

For a negative ExitFailure code, we try to kill the process using that
signal (the details of that are handled by shutdownHaskellAndSignal from
the RTS). For an exit code outside the valid ranges, we use 0xff.
Add tests for the top level exception handler

The top level exception handler is wrapped around main, and FFI exports.
It handles exceptions that are not otherwise caught in user code. For
most exception is just prints them, but handles a few specially,
including ExitCode and UserInterrupt.

On Unix it installs a signal handler for SIGINT to translate it into a
UserInterrupt async exception.

So we test that:

1. receiving SIGINT does trigger a UserInterrupt async exception
2. an unhandled UserInterrupt makes us kill ourselves with SIGINT
3. an unhandled ExitFailure (-sig) makes us kill ourselves with sig