Merge branch 'android-ndk'

Merge branch 'android-ndk'

This branch comes with some preliminary changes for the user-land IPsec
implementation and the Android App.

One important change is that the UDP ports used by the socket-default plugin
were made configurable (either via ./configure or strongswan.conf).
Also, the plugin does randomly allocate a port if it is configured to 0,
which is useful for client implementations.  A consequence of these
changes is that the local UDP port used when creating ike_cfg_t objects has
to be fetched from the socket.