flua: add a chown(2) binding

The main difference with the chown in luaposix, is that it checks
and reports if a user or a group do exist when a string is passed
as arguments

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D37479

(cherry picked from commit a1ab15abe2e2a1ed92cbad056df71afff7e8794e)