dhcp: clean up dhcp4 lease object

dhcp: clean up dhcp4 lease object

a) drop handling of obsolete or unused DHCP options time_offset,
   mtu_aging_timeout, policy filter, mdr, ttl, ip forwarding settings.
   Should this become useful one day we can readd support for this.

b) For subnet mask and broadcast it is not always clear whether 0 or
   255.255.255.255 might be valid, hence maintain a boolean indicating
   validity next to it.

c) serialize/deserialize broadcast address, lifetime, T1 and T2 together
   with the rest of the fields in dhcp_lease_save() and
   dhcp_lease_load().

d) consistently return ENODATA from getter functions for data that is
   missing in the lease.

e) add missing getter calls for broadcast, lifetime, T1, T2.

f) when decoding DHCP options, generate debug messages on parse
   failures, but try to proceed if possible.

g) Similar, when deserializing a lease in dhcp_lease_load(), make sure
   we deal nicely with unparsable fields, to provide upgrade compat.

h) fix some memory allocations