Improve the shutdownHaskellAndSignal and add fast exit

This is the RTS part of a patch to base's topHandler to handle exiting
by a signal.

The intended behaviour is that on Unix, throwing ExitFailure (-sig)
results in the process terminating with that signal. Previously
shutdownHaskellAndSignal was only used for exiting with SIGINT due to
the UserInterrupt exception.

Improve shutdownHaskellAndSignal to do the signal part more carefully.
In particular, it (should) now reliably terminates the process one way
or another. Previusly if the signal was blocked, ignored or handled then
shutdownHaskellAndSignal would actually return!

Also, the topHandler code has two paths a careful shutdown and a "fast
exit" where it does not give finalisers a chance to run. We want to
support that mode also when we want to exit by signal. So rather than
the base code directly calling stg_exit as it did before, we have a
fastExit bool paramater for both shutdownHaskellAnd{Exit,Signal}.
Don translate UserInterrupt into ExitFailure 1, let it propagate

It is important that if a program recieved ^C from the user, and the
program terminates as a result, that it exit with SIGINT so that the
parent process knows that was the case. For example cabal calling ghc
will then be able to tell the difference between it failing and the user
interrupting with ^C and report (or not) appropriately.

So we should just let the UserInterrupt exception propagate to the top
level error handler which will do the right thing.

See http://www.cons.org/cracauer/sigint.html