FUSE: connecting a threaded client before fuse_main hangs the mount

libfuse3's fuse_main daemonizes (forks) unless run with -f. If main() creates a network client before calling fuse_main, any threads that client started (e.g. a reply-reader thread for a pipelined RPC client) don't survive the fork, so the first filesystem call blocks forever (cp hangs in request_wait_answer) while a single-threaded synchronous client happens to work. Fix: in main only probe the server (connect, then disconnect) so errors still reach the terminal, and make the real connection in the .init callback, which runs in the daemonized process. Symptom on the server side: one GETATTR, then the connection closes.