Expose UID_xxx and GID_xxx definitions to userspace.

This patch moves the UID_xxx and GID_xxx definitions out of the
#ifdef _KERNEL section, so that userspace programs like mountd
can use them.
There are a couple of userspace programs that do define UID_ROOT,
but they do not include sys/conf.h. Since they are defined as
the same value, maybe they should be changed to include sys/conf.h.

Reviewed by: kib
Differential Revision: https:/reviews.freebsd.org/D25281