- Move unlinking ipflow from table/list out of ipflow_free().

- Move unlinking ipflow from table/list out of ipflow_free().
- Factor out ipflow_reset() to clear statistics of ipflow to be reused.
- Add flags field in ipflow.  Currently only one flag is defined to make
  sure that the state of the ipflow is correct (on/off the table/list).
- Reference count ipflow, so the ipflow used to do the ifnet.if_output()
  will not be reaped or changed behind our back.  Add commet about it.
- Don't change or reuse an ipflow if it is being actively used,
  i.e. ipf_refcnt > 1
- Fortify ipflow code by putting assertion in proper places.
- Pad ipflow structure explicitly.