C_InitToken: label is not NULL terminated

PKCS#11 v2.2 is very clear that the 'label' argument provided to
C_InitToken is *not* necessarily NULL-terminated, but rather of
fixed size (32 bytes) and whitespace-padded.
Remove deprecated code to pass NULL strings.

Code to pass NULL strings was only used in C_InitToken, but since
the argument there was really space padded and not NULL terminated,
this code is no longer needed.
Add XXX comment about UTF-8 (space strings) in byte buffers.