bin/uuidgen: add support for v4 uuids
The version 4 UUID is meant for generating UUIDs from truly-random or
pseudo-random numbers. [1]
bin/uuidgen gained the new flag '-r' to create version 4 UUID.
[1] RFC 4122, https://www.rfc-editor.org/rfc/rfc4122#section-4.4
Reviewed by: pstef
Approved by: bapt
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37695