network/dhcp4: introduce dhcp4_get_router() helper function

network/dhcp4: introduce dhcp4_get_router() helper function

Previously, we use the first router address, and if it is null, we
ignore the address. Now, we use the first non-null address. That is, if
the first router address is null, but the second is not, then we use the
second one.

That should not cause functional change in most cases, except for the case
when a DHCP server provides such spurious reply.

This is mostly for refactoring and preparation for later commits.