Make malloc more extensible. The malloc type is now a pointer to

Make malloc more extensible.  The malloc type is now a pointer to
the struct kmemstats that describes the type.

This allows subsystems to declare their malloc types locally
and <sys/malloc.h> doesn't need tweaked everytime somebody
gets an idea.  You can even have a type local to a lkm...

I don't know if we really need the longdesc, comments welcome.

TODO: There is a single nit in ext2fs, that will be fixed later,
and I intend to remove all unused malloc types and distribute
the rest closer to their use.