safeLibOp blocks all our vnodesetup-related signals from interrupting
libvnode ops to ensure at least op-level consistency.  However, there
was an opportunity for signals to sneak in, in between a successful
vnodeCreate and the writing of the vnode.info file (that mkvnode.pl uses
to know if the vnode was created or not).

So I redid safeLibOp to make blocking signals optional (of course it's
on for nearly all calls, except now vnodeCreate, and formerly
vnodePoll).  Now there's a signal-safe zone all the way around
vnodeCreate, including a StoreState() before we unblock.  This should
ensure consistency in that particular spot.  I didn't think about
whether this affects anything else.