networkd: Add EmitRouter= option for DHCP Server (#3251)

networkd: Add EmitRouter= option for DHCP Server (#3251)

Add an option to disable appending DHCP option 3 (Router) to the DHCP
OFFER and ACK packets.
This commit adds the boolean option EmitRouter= for the [DHCPServer]
section in .network files.

Rationale: On embedded devices, it is very useful to have a DHCP server
running on an USB OTG ethernet gadget interface to avoid manual setup on
the client PCs, but it should only serve IP addresses, no route(r)s.
Otherwise, Windows clients experience network connectivity issues, due
to them using the address set in DHCP option 3 as default gateway.

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>