If MPTCP send data on multiple ifaces

Thanks to Andrei Maruseac <andrei.maruseac@intel.com>

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>
Change the way routes and DNS are handled

When multipath is enabled we need to ensure that we always have a
default route and a working DNS resolver. This commit ensures that
feature by prefering going through WiFi.

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>
Build fix: change return type

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>
Update MPTCP check

/proc/net/mptcp_pm changed to /proc/net/mptcp.