virt-login-shell: fully reset container environment
    
    The virt-login-shell environment will be initialized with
    an arbitrary number of environment variables determined
    by the SSH daemon and PAM configuration. Most of these are
    not relevant inside the container, and at best they are
    noise and at worst they'll break apps. For example if
    XDG_RUNTIME_DIR is leaked to the container, it'll break
    any apps using it, since  the directory it points to is
    only visible to the host OS filesystem, not the container
    FS.
    
    Use clearenv() to blank out everything and then set known
    good values for PATH, SHELL, USER, LOGNAME HOME and TERM.
    Everything else is left up to the login shell to initialize.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>