ipv4: new default to send gratuitous arp

Consider new suse ifcfg variable default SEND_GRATUITOUS_ARP=auto
for arp-notify, enabling to send gratuitous arp by default, when
duplicate IPv4 check is enabled via arp-verify/CHECK_DUPLICATE_IP
and the check were successful (bnc#879911,fate#314399).
arp: no verify/notify if not applicable for device
ifconfig: fix for an unitialized error return code
Merge pull request #262 from mtomaschewski/auto-garp