LUO: add support for preserving third party sessions

LUO: add support for preserving third party sessions

LUO sessions cannot be nested under other sessions. This means we need
to handle them explicitly, and held them open in the shutdown binary
like we do with our own internal session, to allow services to create
their own.

The requirement to support third party sessions comes from VMMs that
wish to preserve VM(s) state(s) across kexec, as some file descriptors
(KVM's vmfd from the KVM_CREATE_VM ioctl) cannot be transfered between
processes via SCM_RIGHTS, so they cannot be stashed in the FD Store
directly. Also some file descriptors have to be handled all together or
not at all, again to do with KVM and devices that are all part of the
same vm.