GP: add 'internal=no' flag to the login and configuration requests

Without these flags, one user reports consistently hitting the "Matching client config not found" error in response to the /ssl-vpn/getconfig.esp request. See #246

I suspect that 'internal=no' is the implicit default if unspecified (#246 (comment 836128670)), but we should get more testing on other GlobalProtect VPNs to confirm that this works fine with them. If there is variation in whether this parameter is allowed/expected, then we need a way to automatically detect the correct value.