Fix #459

Fix #459

The following changes are made:

- STDOUT and STDIN are redirected to /dev/null when spawning rclone
  rclone blocks and also shows error/warning messages. nnn needs to spawn
  rclone and return without waiting. To avoid the rclone messages from
  corrupting the screen nnn makes the child silent.
  Note: sshfs returns after mount with a proper error code
- prompt only if both sshfs and rclone are installed