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

(cherry picked from commit f176fe8e7f638e585afcd2f4dd52a522c4648f63)