Don't unregister p11-kit PIN callback until vpninfo is finished with

Don't unregister p11-kit PIN callback until vpninfo is finished with

Unregistering in openconnect_close_https() meant that when we reconnect to
the server, we lose the PIN callback. And then when we connect again, if
GnuTLS is asking us for the PIN on every attempt to touch the key, we fail
because there's no PIN handler.

So add a 'final' flag to openconnect_close_https(). Use this to clean up
library.c::openconnect_close_https() a little, too.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>