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.
We've already been through this in this exact pr...

See your comment here https://github.com/Homebrew/homebrew/pull/31196#discussion_r15985227 to which I replied:
> Also no, take a closer look, `make test` runs upstream's full regression suite.