sd-dhcp-relay: introduce new DHCP relay agent

sd-dhcp-relay: introduce new DHCP relay agent

Previously, sd-dhcp-server can be run as a DHCP relay agent.
But, DHCP server and DHCP relay agent behave completely differently,
hence there is almost no code that can be shared between the two modes.
Let's split out the DHCP relay agent feature from sd-dhcp-server.

The new DHCP relay agent supports:
- multiple upstream and downstream interfaces,
- gateway address (giaddr field in DHCP message header) is configurable,
- supports more DHCP relay agent information sub-options,
- each interface has their own socket fd, and each socket is bound to
  the interface, so that we can enable/disable each interface
  safely without affecting other interfaces, and we can filter out any
  unexpected packets from unmanaged interfaces,

networkd integration and test cases will be added later.