Initial MOBIKE Configuration Support

For IKEv2, there is support for MOBIKE which basically allows IPSec connections to roam from interface to interface. When MOBIKE is used, the IKE negoiation phase uses UDP port 4500 rather than using proto-51.

In strongSwan 4.5.x MOBIKE is automatically enabled for IKEv2 connections. We expose the ability to enable/disable MOBIKE to the user.