virt-login-shell: fully reset container environment

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>