Create libstand version of uuid_from_string and uuid_to_string. The
normal libc versions of these functions use asprintf and sscanf, both
of which aren't available in the libstand world. It's easier to
rewrite these than to try to claw those functions into libstand.
Also include uuid_create_nil, since it's safe to include. These were
written from scratch, except for bits of comments that I got from the
libc version, which are acknowledged in the source.

Differential Revision: https://reviews.freebsd.org/D4540