cmd/swarm: FUSE do not require --ipcpath (#18112)

- Have `${DataDir}/bzzd.ipc` as IPC path default.
- Respect the `--datadir` flag.
- Keep only the global `--ipcpath` flag and drop the local `--ipcpath` flag
  as flags might overwrite each other. (Note: before global `--ipcpath`
  was ignored even if it was set)

fixes ethersphere#795