Apparently, it affects whether the prelogin.esp response contains SAML tags
in some cases.
(see https://github.com/dlenski/gp-saml-gui/issues/6#issuecomment-599743060)
This fits in with a long line of mystifying issues caused by GlobalProtect servers
silently handling different `clientos` values in stupidly different ways.
(see https://gitlab.com/openconnect/openconnect/-/merge_requests/17)