ensure that openconnect_disable_{dtls,ipv6} do nothing if vpninfo has ever been connected

ensure that openconnect_disable_{dtls,ipv6} do nothing if vpninfo has ever been connected

Both openconnect_disable_dtls() and openconnect_disable_ipv6() now return
int (0 or -EINVAL), and are used in CLI argument processing.

Ideally, we would allow these to be used when IPv6 or DTLS/ESP are not currently
in use, but we do not have sufficiently reliable cross-protocol indicators of these.
The main use case for both of these functions is disable a problematic feature
prior to initial connection.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>