Initial minimal working tracerestrict implementation.

This is a port of the tracerestrict/routing restrictions feature
from TTDPatch.
At present this implements if tests (train length only),
and pathfinder deny and penalty actions.
This requires the use of YAPF. Note that restrictions are only evaluated
within the YAPF lookahead distance.

src/command_type.h

@@ -329,6 +329,8 @@ enum Commands {
 
 	CMD_OPEN_CLOSE_AIRPORT,           ///< open/close an airport to incoming aircraft
 
+	CMD_PROGRAM_TRACERESTRICT_SIGNAL, ///< modify a signal tracerestrict program
+
 	CMD_END,                          ///< Must ALWAYS be on the end of this list!! (period)
 };