[th/dhcp-plugin-fallback-for-addr-family] dhcp: fallback to internal DHCP plugin if plugin does not support address family
Maybe DHCP plugins should be configurable per address family and be re-loadable via SIGHUP. But that just adds complexity while we actually want to get away from these plugins.
Nowadays we always have the "internal" DHCP plugin, which is known to support both IPv4 and IPv6. One day, we should get rid of all plugins and only use one implementation (that works well). The "internal" plugin is supposed to be that.
That also means, that we are not going to add more (external) DHCP plugins and we are not going to invest much work in the existing plugins (except the "internal" plugin).
Some DHCP plugins are known to not support IPv6. If the user selects "dhcpcd" we should just fallback to the internal plugin. What's the point of letting the activation fail? Probably users shouldn't use "dhcpcd" plugin anyway, but that's a different story. This fallback could be a problem with forward compatibility if we ever would add IPv6 support to "dhcpcd". But we won't.
Also, we are going to add "n-dhcp4" as replacement for the systemd based code. For a time, there will be an experimental plugin "nettools" that eventually will become the new "internal" plugin. Until that happens, we want for IPv6 automatically fallback to systemd based "internal" plugin. This patch will make that simple.