system-linux: add device options used by wpad

system-linux: add device options used by wpad

Add device options used by wpad in preparation of running hostapd and
wpa_supplicant non-root (and hence those options will need to be taken
care of by netifd as sysctl is root-only):
 * drop_v4_unicast_in_l2_multicast
 * drop_v6_unicast_in_l2_multicast
 * drop_gratuitous_arp
 * drop_unsolicited_na
 * arp_accept

Signed-off-by: Daniel Golle <daniel@makrotopia.org>