Kernel: Implement SYS_SYNC and add sync executable to userspace

You can (and have to) manually sync disk after writes to it.

kernel/include/kernel/FS/DevFS/FileSystem.h

@@ -15,6 +15,7 @@ namespace Kernel
 		void initialize_device_updater();
 
 		void add_device(BAN::StringView path, BAN::RefPtr<RamInode>);
+		void for_each_device(const BAN::Function<BAN::Iteration(Device*)>& callback);
 
 		dev_t get_next_dev();