include computer name in the GP cookie

include computer name in the GP cookie

The GlobalProtect "cookie" is an overstuffed monstrosity, due to the
requirement to retain a few random, non-secret values in order to logout
successfully (see gpst_bye):

    authcookie=d41d8cd98f00b204e9800998ecf8427e&portal=Gateway-X&user=user.name&domain=big-corp

Until now, I've avoided including the computer field in this cookie, on the assumption that it
can reproduced at any time using vpninfo->localname. However, it appears that this value can't always
be reproduced correctly when running under NetworkManager:

    https://github.com/dlenski/network-manager-openconnect/issues/7

In order to be more robust, this patch therefore also includes the local hostname in the cookie:

    authcookie=d41d8cd98f00b204e9800998ecf8427e&portal=Gateway-X&user=user.name&domain=big-corp&computer=hostname