AK: Introduce UnixDateTime

This is a generic wrapper for a time instant relative to the unix epoch,
and does not account for leap seconds. It should be used in place of
Duration in most current cases.

Userland/Libraries/LibIPC/Encoder.cpp

@@ -90,6 +90,12 @@ ErrorOr<void> encode(Encoder& encoder, Duration const& value)
     return encoder.encode(value.to_nanoseconds());
 }
 
+template<>
+ErrorOr<void> encode(Encoder& encoder, UnixDateTime const& value)
+{
+    return encoder.encode(value.nanoseconds_since_epoch());
+}
+
 template<>
 ErrorOr<void> encode(Encoder& encoder, URL const& value)
 {