Make it possible for the cdevsw d_close() driver method to detect last
close and close due to revoke(2)-like operation.

A new FLASTCLOSE flag indicates that this is last close. FREVOKE is
set for revokes, and FNONBLOCK is also set, same as is already done
for VOP_CLOSE() call from vgonel().

The flags reuse user open(2) flags which are never stored in f_flag,
to not consume bit space in the ABI visible way. Assert this with the
static check.

Requested and reviewed by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks