uuidgen: add -c for compact uuid

It generates the uuid string but without the hyphen

Approved by: re (cperciva)
MFC After: 3 days
Reviews by: tcberner
Differential Revision: https://reviews.freebsd.org/D38820

(cherry picked from commit b2b294f27cf4618d6f7510007b41882860a080b9)
(cherry picked from commit 0ba66872bea87abdf13660d5a7946769fd7b118f)
(cherry picked from commit 4cd30c640db978355cf2da666e2749189babda5e)