In meshes without VPN or supernodes, it may be desirable to connect a
router providing IPv4 addresses directly to a regular Gluon node. For
this to work, it was necessary to remove the gluon-ebtables-filter-ra-dhcp
package from the node, so the autoupdater also needed to be disabled.

To avoid this, make gluon-ebtables-filter-ra-dhcp disable the DHCPv4
filter rules when gw_mode is set to 'server'.

No solution is provided for RA/DHCPv6, but this seems less pressing for
local meshes, as nodes can already provide working IPv6 connectivity
by themselves through gluon-radvd.