destroyed prior to pfsync_uninit(). To do this, move all the
initialization to the module_t method, instead of SYSINIT(9).
o Fix another panic after module unload, due to not clearing the
m_addr_chg_pf_p pointer.
o Refuse to unload module, unless being unloaded forcibly.
o Revert the sub argument to MODULE_DECLARE, to the stable/8 value.
This change probably isn't correct from viewpoint of VIMAGE, but
the module wasn't VIMAGE-correct before the change, as well.
Glanced at by: bz