pkcs11: Improve detection of already unwrapped CKA_EC_POINTs

pkcs11: Improve detection of already unwrapped CKA_EC_POINTs

If an uncompressed point is already unwrapped (incorrect but some tokens/
modules do this) and therefore still looks like an ASN.1 octet string,
there could be false positives with the previous checks that lead to
mangled points.

By ensuring that we unwrapped the complete ASN.1 chunk, we can already
reduce the false positive rate when the assumed length is smaller than
the chunk, which we previously accepted but isn't the case in correctly
encoded points.

And while we already checked that the first byte indicates a valid point
type/encoding, there could still be false positives.  We can avoid those
with some checks on the length of the unwrapped point.  In particular,
enforcing a multiple of 4/8 should fail for valid unwrapped points where
three bytes were removed in the process (ASN.1 tag and length, point
encoding).