devel/efivar: Update to version 39 and take maintainership

This is a big and important update which was initially submitted as
part of D29332 by Norbert Kaminski <norbert.kaminski@3mdeb.com>. The
submitted patches were for 0.24 and at the last two EuroBSDCons I've
spend a lot of time to rework them for v39. Thanks a lot for the
initial patches, even if there is not much of the initial work left.

Differential Revision: https://reviews.freebsd.org/D29332