qmi: add metric, defaultroute and peerdns options for qmi protocol

Adds generic network options for qmi protocol dynamic interfaces
as suggested by Felix in
https://lists.openwrt.org/pipermail/openwrt-devel/2016-February/039794.html.

IPv6-related code taken from Bruno's patch https://patchwork.ozlabs.org/patch/584816.

This depends on netifd patch https://patchwork.ozlabs.org/patch/686820/

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Signed-off-by: Marcin Jurkowski <marcin1j@gmail.com>
Signed-off-by: Bruno Randolf <br1@einfach.org>