Change the deadfs poll VOP to return POLLIN|POLLRDNORM if the caller
is interested in i/o state.  Return POLLNVAL for invalid bits, similar
to poll_no_poll().  Note that POLLOUT must not be returned, since
POLLHUP is set.

Noted and reviewed by:	bde
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
When vnode bypass cannot be performed on the cdev file descriptor for
read/write/poll/ioctl, call standard vnode filedescriptor fop.  This
restores the special handling for terminals by calling the deadfs VOP,
instead of always returning ENXIO for destroyed devices or revoked
terminals.

Since destroyed (and not revoked) device would use devfs_specops VOP
vector, make dead_read/write/poll non-static and fill VOP table with
pointers to the functions, to instead of VOP_PANIC.

Noted and reviewed by:	bde
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week