Swiftly for OpenBSD

[grynspan]

Although OpenBSD is not officially supported by Swift at this time, if it ever is we should make sure swiftly supports it too.

Some notes:

• OpenBSD uses pkg_add to install packages from its official package repository.
• The set of dependencies is roughly the same as for Linux or FreeBSD.
• libuuid is still part of e2fsprogs rather than a standalone package.
• Swift Testing needs the zip package so it can correctly generate directory attachments.