Add space before "]" in "if [ ... ]" to fix usage error
and test for existence of the correct file.
When creating a vnode with vcache_get() mark the vnode VI_CHANGING until
it is fully initialised.  It may be on the specnode list before it is
fully initialised and revoking it then would panic.

Should prevent the panic from PR kern/49171 (panic when closing a pty).