sd-dhcp-server: also send DNS servers or friends on DHCPOFFER

sd-dhcp-server: also send DNS servers or friends on DHCPOFFER

From RFC 2131 section 4.3.1 (https://www.rfc-editor.org/rfc/rfc2131#section-4.3.1):
----
The server MUST return to the client:
- Parameters requested by the client, according to the following rules:
  -- IF the server has been explicitly configured with a default
     value for the parameter, the server MUST include that value
     in an appropriate option in the 'option' field,
----
The sentence is not only for ACK, but for all (positive) responses, that
is DHCPOFFER and DHCPACK.

Fixes #27471.