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.
Except to run the regression suite I need the context which seems to only be available while building, which is why I asked about it [here](https://github.com/Homebrew/homebrew/issues/30868) before submitting this pull request.

Do you have a better answer to that question?