POSIX/FreeBSD/NetBSD/Linux/etc agree -- blk[size|cnt]_t is the bomb.

So add the types blksize_t (a.k.a. int32_t) and blkcnt_t (a.k.a.
int64_t). Use blkcnt_t in stat.h since the base type (int64_t) does
not change. blksize_t in stat.h will follow after the tree is audited
for signed issues, since the base type will change from u_int32_t
to a POSIX compliant int32_t.

Guidance and feedback from guenther@

ok millert@