Update README quick install for Fedora

- Use systemctl start ensure autofs is started and enabled
- Use 'su -c' instead of sudo, since sudo is not configured for regular
  users by default on Fedora
Use groupadd/groupdel as fallback in install

- Fedora uses groupadd/groupdel for groups management
- Use of -f (force) flag means install target doesn't fail if group
  already exists (e.g. if running install a second time)
Use systemctl to restart autofs

- Fedora uses systemd
Use gpasswd to add/remove user from group

- adduser under Fedora is for adding users not modifying existing ones
- gpasswd appears to be available on Ubuntu as well as Fedora so need to
  add "fallbacks"