IWYU update

- Add Settings forward declaration in Process.h
- Add assert.h include in XUitls.c
- Add conditional stdbool.h include in Object.h
- Drop unneeded stddef.h include in Richstring.c
- Drop unneeded unistd.h include in Process.h
- Drop unneeded string.h include in linux/Platform.c
- Use String_eq to avoid string.h include in Action.c
- Improve script to run custom iwyu version

linux/Platform.c

@@ -13,7 +13,6 @@ in the source distribution for its full text.
 #include <ctype.h>
 #include <math.h>
 #include <stdio.h>
-#include <string.h>
 
 #include "BatteryMeter.h"
 #include "ClockMeter.h"