add signify

This is a port of the [signify(1)](http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man1/signify.1) utility from OpenBSD.
That's not very comprehensive. There's a reason I included the infrastructure to run upstream's regression suite, is there no way to access the required files in a test block? Would you actually prefer a simple test like the above? If so I'd be happy to provide one, but it seems counterproductive, especially considering much better tests are already available.