* add check_site.lua to validate required field from site
* make use of node_client_prefix6 if defined in site

uc is used in the initscript. It must be set as dependency

This patch makes use of the new feature in l3roamd to gracefully
add, remove and list the mesh interfaces that are currently in use. This
helps when changing mesh interfaces often - a characteristic of the
wireguard protocol implementation as in the previous behavior all local
clients are dropped when adjusting mesh interfaces.

The local_node ifstatus must be checked for local-node, not client. While
we're at it, also clean up the syntax a bit.

gluon-core, gluon-l3roamd: introduce script gluon-list-mesh-interfaces that lists all currently active mesh interfaces

* use local socket
* use main routing table,
* pass client-bridge

 * gluon-core, gluon-client-bridge: introduce new firewall zone: local_client
 * gluon-core: put clients in local_client zone, introduce drop-zone,
   set dns-rules and zones
 * gluon-respondd: allow respondd on mesh
 * gluon-status-page-api: allow http input on mesh and client

Some files have received some additional refactoring.

gluon-mesh-babel is not merged yet, and once it is, this would lead to a
circular dependency.