Add VM_EXITCODE_IPI to permit returning unhandled IPIs to userland. INIT and Startup IPIs are now returned to userland. Due to backward compatibility reasons, a new capability is added for enabling VM_EXITCODE_IPI. MFC after: 2 weeks Differential Revision: https://reviews.freebsd.org/D35623 Sponsored by: Beckhoff Automation GmbH & Co. KG