udhcpd: keep expired leases at startup

udhcpd: keep expired leases at startup

Let udhcpd retain the information about expired leases when restarting
so that the leases are reserved until they possibly become the oldest
expired lease.

This reduces the frequency of IP address changes for example when the
DHCP server and a group of clients, who do not store and request their
previously offered IP address across restarts, are collectively restarted
and the startup order of the clients are not guaranteed.

Signed-off-by: Christian Lindeberg <christian.lindeberg@axis.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>