specfs: Split spec_open switch into three sections.

The sections are now:

1. Acquire open reference.

1a (intermezzo). Set VV_ISTTY.

2. Drop the vnode lock to call .d_open and autoload modules if
  necessary.

3. Handle concurrent revoke if it happenend, or release open reference
  if .d_open failed.

No functional change.  Sprinkle comments about problems.

(riastradh)