Add optional device vnode bypass to DEVFS.

The tunable vfs.devfs.fops controls this feature and defaults to off.

When enabled (vfs.devfs.fops=1 in loader), device vnodes opened
through a filedescriptor gets a special fops vector which instead
of the detour through the vnode layer goes directly to DEVFS.

Amongst other things this allows us to run Giant free read/write to
device drivers which have been weaned off D_NEEDGIANT.

Currently this means /dev/null, /dev/zero, disks, (and maybe the
random stuff ?)

On a 700MHz K7 machine this doubles the speed of
	dd if=/dev/zero of=/dev/null bs=1 count=1000000

This roughly translates to shaving 2usec of each read/write syscall.

The poll/kqfilter paths need more work before they are giant free,
this work is ongoing in p4::phk_bufwork

Please test this and report any problems, LORs etc.

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