x64: skip unformated sections when iterating through dmi table

DMI structure consists of formated and unformated sections. Only the size
of the former is stored in the header. The end of the unformated section
is denoted by two consecutive 0.

Fixes #521.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Implement chmod()

Implement the chmod() function, using the existing low-level vnode
(and ultimately, zfs) mechanisms. Also added a test to verify that
after chmod, stat returns the expected mode.

Fixes #523.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>