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

Process.h

@@ -9,7 +9,6 @@ in the source distribution for its full text.
 */
 
 #include <stdbool.h>
-#include <unistd.h>
 #include <sys/types.h>
 
 #include "Object.h"
@@ -55,6 +54,8 @@ typedef struct ProcessPidColumn_ {
    const char* label;
 } ProcessPidColumn;
 
+struct Settings_;
+
 typedef struct Process_ {
    Object super;