SDL Sopwith v2.2.0

• Multiplayer support was added for the Windows version, at long last.
• Help text is now shown for new players. This is shown on game start in
  novice mode, but won't be shown again after a successful flight. The help
  text will also appear in other single player modes, but only if it looks
  like you're really struggling to get the plane off the ground.
• It's no longer possible to flip the plane upside down while sitting
  motionless on the runway (a bug since the original DOS versions).
• It's no longer possible to get wounded on the ground. If a plane shoots
  another on the ground, that's a successful raid. This fixes a bug where
  a wounded plane on the runway would still be damaged after takeoff.
• The game no longer freezes up in multiplayer mode if the other player
  quits. When establishing a connection, a helpful message is shown about how
  to abort.
• An extra help doc was added for macOS users.
• If the game quits with an error, a friendly dialog box now shows the error.
• Some minor manpage fixes: the deleted missiles mode flag is no longer
  mentioned, and the spacing appears correct on OpenBSD man(1).
• Lots more internal cleanups making good progress towards eventually adding
  support for custom levels.