if_me: Use dedicated network privilege

Separate if_me privileges from if_gif.

Reviewed by: kp
Approved by: kp (mentor)
Differential Revision: https://reviews.freebsd.org/D36691

(cherry picked from commit 43f8c763cdeea29f95b6f0eebce3ad80dd210c7a)
(cherry picked from commit 310f6f2448ed5baf0d74fba7660468df84dce184)