sd-dhcp-client: fix REBOOT state handling

client->secs wasn't getting set in the REBOOT state, causing
an assertion.  REBOOT should work the same way as INIT, per
RFC 2131:

 secs   2  Filled in by client, seconds elapsed since client
           began address acquisition or renewal process.

REBOOT is necessary because some DHCP servers (eg on
home routers) do not hand back the same IP address unless the
'ciaddr' field is filled with that address, which DISCOVER
cannot do per the RFCs.  This leads to multiple leases
on machine reboot or DHCP client restart.