Merge r261590, r261592 from head:
  Remove identical vnet sysctl handlers, and handle CTLFLAG_VNET
  in the sysctl_root().

  Note: SYSCTL_VNET_* macros can be removed as well. All is
    needed to virtualize a sysctl oid is set CTLFLAG_VNET on it.
    But for now keep macros in place to avoid large code churn.
Merge r261590: Fixup for r261590 (vnet sysctl handlers cleanup)