rdisc,device: Set MTU if an appropriate option is present in a RA

https://bugzilla.gnome.org/show_bug.cgi?id=738104

Reported-by: Charles R. Anderson <cra@wpi.edu>
Merge remote-tracking branches 'lr/lr-unmanaged-slaves', 'lr/lr-ipv6-tokens', 'lr/lr-ap' and 'lr/lr-rdisc-mtu' into integration