zcip: use low-order 4 bytes of MAC as random seed, not 4 high-order

zcip: use low-order 4 bytes of MAC as random seed, not 4 high-order
arping: fix wrong roundtrip calculation
arping,zcip: cleanups and code shrink

run                                                  389     402     +13
arp                                                  195     188      -7
zcip_main                                           1524    1495     -29
arping_main                                         1874    1823     -51
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/3 up/down: 13/-87)            Total: -74 bytes
   text    data     bss     dec     hex filename
 776587     929    9100  786616   c00b8 busybox_old
 776499     929    9100  786528   c0060 busybox_unstripped