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

(cherry picked from commit bca9c87b6104219af35ae5ea4a6af098a1631bca)