net: Add ip rule save/restore

Add support for save and restore of ip rules. It uses new
functionality of iproute which is already in iproute git:

http://git.kernel.org/cgit/linux/kernel/git/shemminger/iproute2.git/commit/?id=2f4e171f7df22107b38fddcffa56c1ecb5e73359



v2: Use xstrdup() instead of strdup().
v3: Use open/close instead of helper.
v4: Return -1 on empty dump.

Signed-off-by: Kirill Tkhai <ktkhai@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>