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.
