stoken: Implement new auth form to gather soft token information

If the user has asked to use a soft token, libopenconnect will prompt
for devid/pass/pin (as necessary) to unlock the soft token, prior to
the initial server connection.  If the user aborts, soft token mode will
be disabled and the user will need to enter his tokencode by hand.
Manual entry could be useful for e.g. activating a new token.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>