9p: Add 'u' and 'g' format specifies for kuids and kgids

This allows concentrating all of the conversion to and from kuids and
kgids into the format needed by the 9p protocol into one location.

Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@gmail.com>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>