stand: Move md_copymodules into modinfo.c and reduce copies

md_copymodules, bi_copymdoules, bi_copymodules32 (x2) and
bi_copymodules64 (x2) are all the same routine... Replace them all with
md_copymodules. This saves about 800 bytes on i386 BIOS loader, which is
a nice bonus.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36572