tree-wide: beautify remaining copyright statements

Let's unify an beautify our remaining copyright statements, with a
unicode ©. This means our copyright statements are now always formatted
the same way. Yay.

src/basic/bitmap.c

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 /***
-  Copyright 2015 Tom Gundersen
+  Copyright © 2015 Tom Gundersen
 ***/
 
 #include <errno.h>