nixos/initrd-network: multiple fixes

 * acquire DHCP on the interfaces with networking.interface.$name.useDHCP == true or on all interfaces if networking.useDHCP == true (was only only "eth0")
 * respect "mtu" if it was in DHCP answer (it happens in the wild)
 * acquire and set up staticroutes (unlike others clients, udhcpc does not do the query by default); this supersedes https://github.com/NixOS/nixpkgs/pull/41829