(svn r1616) Introduce ttd_strlcat() and use it to de-uglify some piece of code in misc_cmd.

While here rename the len parameter of ttd_strlcpy() to size, because it is a buffer size and not a string length.
Also add -Wwrite-strings to the Makefile, because the above mentioned piece of code was the only part which triggered this warning.

functions.h

@@ -271,7 +271,8 @@ enum {
 };
 void ShowSaveLoadDialog(int mode);
 
-void ttd_strlcpy(char *dst, const char *src, size_t len);
+void ttd_strlcpy(char *dst, const char *src, size_t size);
+void ttd_strlcat(char *dst, const char *src, size_t size);
 
 // callback from drivers that is called if the game size changes dynamically
 void GameSizeChanged(void);