openzfs: Add a _WANT_ZNODE that defines struct znode / znode_t for userspace

Note that ZNODE_OS_FIELDS needs to change to using struct vnode over
vnode_t (matching struct zfsvfs rather than vnode_t) since vnode_t is
only defined in the kernel SPL, not the userspace SPL (libspl).

Whilst here, tidy up the includes and clarify a comment.

Reviewed by: imp, markj
Differential Revision: https://reviews.freebsd.org/D50720