LibMain: Add the ability to configure the exit code on error

Some POSIX utilities are specified to return a specific value on error,
which is not 1. `Main::set_return_code_for_errors()` lets you set it to
that value.

Userland/Libraries/LibMain/Main.h

@@ -19,6 +19,9 @@ struct Arguments {
     Span<StringView> strings;
 };
 
+int return_code_for_errors();
+void set_return_code_for_errors(int);
+
 }
 
 ErrorOr<int> serenity_main(Main::Arguments);