Kernel: Use UnixDateTime wherever applicable

"Wherever applicable" = most places, actually :^), especially for
networking and filesystem timestamps.

This includes changes to unzip, which uses DOSPackedTime, since that is
changed for the FAT file systems.

Kernel/FileSystem/SysFS/Inode.cpp

@@ -110,7 +110,7 @@ ErrorOr<void> SysFSInode::truncate(u64 size)
     return m_associated_component->truncate(size);
 }
 
-ErrorOr<void> SysFSInode::update_timestamps(Optional<Duration>, Optional<Duration>, Optional<Duration>)
+ErrorOr<void> SysFSInode::update_timestamps(Optional<UnixDateTime>, Optional<UnixDateTime>, Optional<UnixDateTime>)
 {
     return {};
 }