stand: collapse all copies of *copyenv into md_copyenv

Use the efi's bi_copyenv to md_copyenv and place it in modinfo.c. Remove
all other nearly identical and efi's has the best error handling.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36574

(cherry picked from commit fc352701ff3aeb0af22c0da17c4194cf1f8ad5d0)