sys/dev/disk/dm: Don't define target specific defs in dm.h

sys/dev/disk/dm: Don't define target specific defs in dm.h

dm targets should not have target local structures, function
prototypes, etc in dm.h which is a header for dm core.
It has no dependency on a certain dm target, and works with
or without a certain dm target.

Targets local structures are usually private data part of dm,
and target local functions are called via pointers. dm core
doesn't recognize these by names or types.

NetBSD had (and still has) it this way, but this change will
not make it any difficult to sync with NetBSD in the future
since NetBSD also isn't likely to use them in dm core by its
design. They're just defined there.