library: Allow overriding cached PIN

If ctx->t->pin already contains a PIN, stoken_pin_required() returns
false to indicate that the caller doesn't need to pass in a PIN.  However,
we don't want to use this check inside stoken_compute_tokencode() to
decide whether to ignore the user-supplied PIN string, as he may want
the computation to use a different PIN.

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