android: Add a custom kernel-net implementation to replace kernel-netlink

When roaming from a mobile network to WiFi on Android 5.x the event
received via ConnectivityManager is triggered before the mobile
connection is fully torn down (i.e. before the interface is disabled and
the routes disappear).  So for strongSwan the current path still seems
valid and since no roam event is triggered later the daemon never switches
to WiFi and the connection is broken afterwards.

A possible solution to this is enabling roam events in the kernel-netlink
plugin.  That would trigger an event when the device is finally disconnected
from the mobile network.  However, this could actually take a some time,
during which traffic continues to be sent via mobile network instead of WiFi.
That's because Android now uses multiple routing tables, routing rules and
fwmarks to direct traffic to the appropriate interface/table, but in our
plugin we don't have the information available that would allow us to make
the switch to a different network/routing table earlier (and we actually
prefer the current path if it is still valid).  Additionally, the plugin
produces quite a bit more events than ConnectivityManager (which was one
of the reasons to use the latter in the first place).

This custom kernel-net implementation is now specifically tailored for
Android.  Roam events are still triggered via ConnectivityManager but
the source address is determined via connect()/getsockname() on a VPN
excluded UDP socket, which does use the correct routing table as intended
by Android.  That way the daemon immediately sees a different source IP
when connectivity changes even if the device is connected to multiple
networks concurrently.

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