terminal: abstract terminal color handling

Instead of making msg.c an ifdef hell for unix vs. windows code, move
the code to separate functions defined in terminal-unix.c/terminal-
win.c.

Drop the code that selects random colors for --msgmodule prefixes.

player/main.c

@@ -264,7 +264,7 @@ static void osdep_preinit(int *p_argc, char ***p_argv)
     SetErrorMode(0x8003);
 #endif
 
-    load_termcap(NULL); // load key-codes
+    terminal_init();
 
     mp_time_init();
 }