stand: update prototypes for md_load and md_load64

These are declared as extern in a number of files (some with the wrong
return type). Centralize this in modinfo.h and remove a few extra stray
declarations as well that are no longer used. No functional change.

Note: I've not tried to cope with the bi_load() functions which are the
same logical thing. These will be handled separately.

Sponsored by: Netflix

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

stand: Move MOD_xxx macros from modinfo.h to .c

Now that MOD_xxx macros are modinfo.c, they don't need to be in
modinfo.h.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36573

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

stand: Create common/modinfo.h

Move all the MOD_xxx macros to this header. Each user of this interface
is currently required to define MOD_ALIGNMENT(l). modinfo was selected
because it sits inbetween modules and metadata and will make it easier
to migrate to new, shared intefaces.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36571