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

(cherry picked from commit 335615c4cad79765b46048bfa782120fd9cadb95)