Use the vnode v_opencount and v_writecount universally. They were previously

Use the vnode v_opencount and v_writecount universally.  They were previously
only used by specfs.  Require that VOP_OPEN and VOP_CLOSE calls match.
Assert on boundary errors.

Clean up umount's FORCECLOSE mode.  Adjust deadfs to allow duplicate closes
(which can happen due to a forced unmount or revoke).

Add vop_stdopen() and vop_stdclose() and adjust the default vnode ops to
call them.  All VFSs except DEADFS which supply their own vop_open and
vop_close now call vop_stdopen() and vop_stdclose() to handle v_opencount
and v_writecount adjustments.

Change the VOP_OPEN/fp specs.  VOP_OPEN (aka vop_stdopen) is now responsible
for filling in the file pointer information, rather than the caller of
VOP_OPEN.  Additionally, when supplied a file pointer, VOP_OPEN is now
allowed to populate the file pointer with a different vnode then the one
passed to it, which will be used later on to allow filesystems which
synthesize different vnodes on open, for example so we can create a generic
tty/pty pairing devices rather than scanning for an unused pty, and so we
can create swap-backed generic anonymous file descriptors rather than having
to use /tmp.  And for other purposes as well.

Fix UFS's mount/remount/unmount code to make the proper VOP_OPEN and
VOP_CLOSE calls when a filesystem is remounted read-only or read-write.