stand: create common set_currdev

Pull together the nearly identical copies of set_currdev in i386,
userboot and efi. Other boot loaders have variances that might be fine
to use the common routine, or not. Since they are harder to test for me,
and ofw and uboot do handle these setting differently, leave them be for
now.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38005