Change: [Linkgraph] Pause the game when linkgraph jobs lag (#6470)

Check if the job is still running two date fract ticks before it is due
to join, and if so pause the game until its done.
When loading a game, check if the game would block immediately due to
a job which is scheduled to be joined within two date fract ticks,
and if so pause the game until its done.
This avoids the main thread being blocked on a thread join, which appears
to the user as if the game is unresponsive, as the UI does not repaint
and cannot be interacted with.
Show if pause is due to link graph job in status bar, update network
messages.
This does not apply for network clients.

src/openttd.h

@@ -62,6 +62,7 @@ enum PauseMode : byte {
 	PM_PAUSED_ERROR          = 1 << 3, ///< A game paused because a (critical) error
 	PM_PAUSED_ACTIVE_CLIENTS = 1 << 4, ///< A game paused for 'min_active_clients'
 	PM_PAUSED_GAME_SCRIPT    = 1 << 5, ///< A game paused by a game script
+	PM_PAUSED_LINK_GRAPH     = 1 << 6, ///< A game paused due to the link graph schedule lagging
 
 	/** Pause mode bits when paused for network reasons. */
 	PMB_PAUSED_NETWORK = PM_PAUSED_ACTIVE_CLIENTS | PM_PAUSED_JOIN,