Remove `sc_dead' logic from pppac(4). It is used to prevent

Remove `sc_dead' logic from pppac(4). It is used to prevent
pppac_ioctl() be called on dying pppac(4) interface. But now if_detach()
makes dying `ifp' inaccessible and waits for references which are in-use
in ioctl(2) path. This logic is not required anymore. Also if_detach()
was moved before klist_invalidate() to prevent the case while
pppac_qstart() bump `sc_rsel'.

ok yasuoka@