add PAN GlobalProtect protocol support (HTTPS tunnel only)

add PAN GlobalProtect protocol support (HTTPS tunnel only)

This includes support for generating an xmlconfig so that NetworkManager can
list all the gateway servers, when connecting to a GlobalProtect portal (not
gateway):

GlobalProtect distinguishes "portal" and "gateway" servers.  Often the same
server supports both (/global-protect URLs are for the portal, /ssl-vpn URLs
are for the gateway).  The official clients always connect through the
portal.  Mostly, the portal configuration is not useful for OpenConnect; it
restricts the behavior of the official clients.

However, the portal configuration does contain a list of allowed gateways
(just as AnyConnect VPNs can list other servers).  We therefore generate an
xmlconfig in the same format as AnyConnect VPNs, so that the NetworkManager
plugins can list all the supported gateways.

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