nspawn: set up a new session keyring for the container process

nspawn: set up a new session keyring for the container process

keyring material should not leak into the container. So far we relied on
seccomp to deny access to the keyring, but given that we now made the
seccomp configurable, and access to keyctl() and friends may optionally
be permitted to containers now let's make sure we disconnect the callers
keyring from the keyring of PID 1 in the container.