Revert "Use Mono.Unix.UnixSignal for handling SIGINT."

This reverts commit 4f7b796482e926bf6156bf460ce7aa510f4ad44e.
Use Mono.Unix.UnixSignal for handling SIGINT, take two.

Like 4f7b796482e926bf6156bf460ce7aa510f4ad44e, but:

* The EnableControlC configuration option still works.
* It's now more clear where the Ctrl-C handlers are set and unset.
* SIGINT is now properly ignored when the signal handler is inactive.