efibootmgr: provide explicit error message if run as non-root

This should be less confusing for new users.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42213

(cherry picked from commit c08ba4a588d517e3c5fe00cee0927294d2490993)
(cherry picked from commit 01641167c9660e98122c3f6174878d534a83b57e)