add oncp_bye() to logout the Juniper session

add oncp_bye() to logout the Juniper session

The nc protocol lacked a .vpn_close_session function; without logout, the
VPN cookie remains active and can be used to restart the session, which is a
security hazard—especially when passing around OpenConnect logs on the
mailing list for development and troubleshooting.

Juniper logout is straightforward: GET /dana-na/auth/logout.cgi (with the
appropriate DSID cookie set).

Signed-off-by: Daniel Lenski <dlenski@gmail.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>