vfs: implement futimesat()

Fixes #330

OpenJDK uses futimesat() in some places.
dirfd must point to a directory if pathname is not absolute.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>