Initialize va_filerev in vattr_null() to avoid leaking stack garbage;

Initialize va_filerev in vattr_null() to avoid leaking stack garbage;
problem pointed out by Martin Natano (natano (at) natano.net)

Also, stop chaining assignments (foo = bar = baz) in vattr_null().
The exact meaning of those depends on the order of the sizes-and-
signednesses of the lvalues, making them fragile: a statement here
mixed *six* types, but managed to get them in a safe order.  Delete
a 20+ year old XXX comment that was almost certainly bemoaning a bug
from when they were in an unsafe order.

ok deraadt@ miod@