flua: Add wrappers for sys/utsname.h

This allows one to invoke uname from lua scripts.

Reviewed by: bapt, kevans, emaste
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D42017

(cherry picked from commit 1726db7af6b3738eb04d962b351d7f4017e1fc77)