library: Force stoken_encrypt_seed() to create v2 tokens

If a v3 token is imported, then stoken_encrypt_seed() is called, it will
try to create a v3 token.  This is a problem for two reasons: first, it
is unexpected behavior from the caller's perspective; second, it causes
a heap overflow because only (MAX_TOKEN_CHARS + 1) bytes were allocated
to store the string.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
securid: Centralize pass/devid sanity checks for securid_decrypt_seed()

Use common logic for v2/v3/sdtid tokens where possible.  This addresses a
problem where e.g. non-devid-bound v3 tokens failed decryption if a devid
was specified.

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