Slight test speedup

Don't do a pointless ARP check in dhcpcd.
Work around apparent dhcpcd bug
Pull in nix-daemon.socket

This led to the container test failing, which made no sense
whatsoever, until I realized nix-daemon.socket creates the socket
directory as a side effect, which systemd-nspawn then bind-mounts.

http://hydra.nixos.org/build/10397575
kmod-static-nodes: Drop superfluous wantedBy