Add configurable uBridge control channel transport (tcp/unix)

Add ubridge_control_transport to [Server] config (default "tcp", fully
backward compatible). Selecting "unix" switches the uBridge hypervisor
control channel from the unauthenticated TCP listener (-H) to an AF_UNIX
socket (-U) authenticated in-kernel via SO_PEERCRED.

- UBridgeHypervisor: supports both socket_path (AF_UNIX) and host/port
  (TCP); a new `endpoint` property unifies log/error strings for both
- Hypervisor: unix mode allocates a short socket path under a 0700
  private runtime dir and unlinks it on stop; tcp mode restores the
  original getaddrinfo ephemeral-port allocation
- base_node: reads the transport from config and passes host through
- schemas/config.py + config_samples/gns3_server.conf: new option

Default deployments are unchanged. "unix" requires a ubridge build that
understands -U.

gns3server/config_samples/gns3_server.conf

@@ -92,6 +92,11 @@ udp_end_port_range = 30000
 ; uBridge executable location, default: search in PATH
 ;ubridge_path = ubridge
 
+; uBridge control channel transport: "tcp" (-H host:port, default) or "unix"
+; (-U socket_path; AF_UNIX + SO_PEERCRED, recommended on Linux for kernel-level
+; peer authentication). TCP now binds loopback by default.
+;ubridge_control_transport = tcp
+
 ; Marker (traffic-insight) UDP sink: one listener per compute process that
 ; receives uBridge MARK signals from every uBridge on this host.
 ; marker_listen_host defaults to 127.0.0.1 because uBridge runs locally.