UUID generator (RFC 4122), see <https://www.rfc-editor.org/rfc/rfc4122>

It generates UUID versions 1, 3, 4, 5, 6, 7 and 8, and nil UUID.

It returns the URN; if you want to emulate the same behaviour as uuigen (plain
UUID), set the env-var:

export UUID_MODE=uuidgen
