Standardize the definition of a UFS dinode.

Each program that operates on UFS on-disk inodes defines its own
version of a dinode. They all (of necessity) define the same
layout but use different names. This change adds a definition of
a dinode (a union of a UFS1 on-disk inode and a UFS2 on-disk inode)
as well as a dinodep (a union of a pointer to a UFS1 on-disk inode
and a pointer to a UFS2 on-disk inode) in sys/ufs/ufs/dinode.h.
It then deletes the definitions of dinode and dinodep in all the
programs that operate on them and instead uses these standard
definitions.

No functional change intended.

MFC-after: 1 week