Do not enter a tight INIT -> REQUESTING -> INIT loop when the dhcp

Do not enter a tight INIT -> REQUESTING -> INIT loop when the dhcp
server responds to our DHCPDISCOVER but is then slow to respond to our
DHCPREQUEST.
MAX_EXP_BACKOFF_FAST was introduced to get us quickly out of the
REBOOTING state when we switch networks and no dhcp server would NAK
our old lease but just ignore us. This is not the issue here, there is
a dhcp server willing to talk to us, it's just slow.
Problem reported, tested & OK jca