odhcp6c: fix parse errors with some dhcpv6 servers

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42869 3c298f89-4303-0410-b956-a3cf2f4a3e73
netifd: Make mtu configurable of dynamic 6rd tunnel interface

Patch allows to configure the mtu of the dynamic 6rd tunnel interface when created by dhcp script.
In some setups it's desirable to have config control over the 6rd tunnel mtu to maximize the traffic throughput

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42871 3c298f89-4303-0410-b956-a3cf2f4a3e73