dhcp4: omit arp validation if address is on link
dhcp4: modprobe af_packet module at start

In hope this makes the gap after link-up shorter.
dhcp4: check if link is up before start

Also try a quick renew of existing lease at start
dhcp4: do not discard lease if host does not match

The dhcp server may return the requested hostname
or completely another one or no hostname at all.
dhcp4: confirm lease after link change or restart

Dropped fail_on_timeout flag and renamed fsm_quick_renewal to
fsm_reboot to confirm lease using broadcast as defined in the
RFC2131 (3.2, reusing a previously allocated network address).