cleanup: Reduce the amount of "extern" in .c files.

There is no check at all that the types of externs actually match the
type of the definition. This just accidentally was not a problem, but now
the compiler helps a bit in keeping it that way.

src/settings.h

@@ -96,6 +96,8 @@ struct user_settings {
 #endif
 };
 
+extern struct user_settings *user_settings;
+
 enum settings_values {
     AUTOLOG_OFF = 0,
     AUTOLOG_ON = 1,