sd-dhcp-client: make timeout handling a bit more robust

Accept any lease lifetime greater than one second. Server should not
hand out extremely short leases, but let's not be the ones to fail.

Do not fail when arming a timer in the past, but also only arm one such
timer.

Avoid rounding errors when computing the default timeouts, this may be
an issue if we are handed a very short lease.

Also, don't pass 'time_now' around, as that can be found in the event
object when needed.
sd-dhcp-client: add fallback subnet masks

The DHCP RFC does not require the DHCP server to send a subnet mask, so if it
is missing, let's try to use the default subnet masks based on address class.
In case the class the address belongs to does not have a default subnet mask,
we fail as before.

Also improve logging when handling invalid dhcp messages, and simply ignore them
rather than stop the whole dhcp client.
network: dhcp: create explicit host route to gateway

Some DHCP servers gives you a netmask of 255.255.255.255 so the gateway is not
routable. Other DHCP client implementations look through the existing routes to
figure out if they should add an explicit host route. See below for a link.

However, it makes sense to just create the route explicitly whether it is
needed or not since it is explicit, makes the dhcp route entries independent of
other entries and saves us from knowing the state of the kernel tables.

After patch route table on a machine with a network (common case):

default via 10.0.2.2 dev ens3
10.0.2.0/24 dev ens3  proto kernel  scope link  src 10.0.2.15
10.0.2.2 dev ens3  scope link

After patch route table on a machine without a network (this case):

default via 10.240.0.1 dev ens4v1
10.240.0.1 dev ens4v1  scope link

The code from dhcpcd that works around this issue is on line 637.
https://android.googlesource.com/platform/external/dhcpcd/+/master/configure.c