(svn r1453) Feature: MD5 hash check for TTD files

The original TTD files are now checked with a MD5 sum to determine which version of the grf files is used and to warn about possible file corruptions. (Thanks to ledow for the original patch)

ttd.c

@@ -545,6 +545,7 @@ void LoadIntroGame()
 }
 
 extern void DedicatedFork(void);
+extern void CheckExternalFiles();
 
 int ttd_main(int argc, char* argv[])
 {
@@ -638,6 +639,7 @@ int ttd_main(int argc, char* argv[])
 	}
 
 	DeterminePaths();
+	CheckExternalFiles();
 
 #ifdef UNIX
 	// We must fork here, or we'll end up without some resources we need (like sockets)