bug: `is_fn_supported()` always returns `true`

[arjennienhuis]

is_fn_supported() returns true for all libraries and functions I tested with. This makes sense when reading the spec:

• http://docs.oasis-open.org/pkcs11/pkcs11-base/v2.40/os/pkcs11-base-v2.40-os.html
• http://docs.oasis-open.org/pkcs11/pkcs11-base/v3.0/pkcs11-base-v3.0.html

Both say:

Every function in the Cryptoki API MUST have an entry point defined in the Cryptoki library’s CK_FUNCTION_LIST structure. If a particular function in the Cryptoki API is not supported by a library, then the function pointer for that function in the library’s CK_FUNCTION_LIST structure should point to a function stub which simply returns CKR_FUNCTION_NOT_SUPPORTED.

[ionut-arm]

Somewhat late on this one, but given how many implementations treat compliance with the spec merely as a "nice to have", it's probably safer to keep the check..

[hug-dev]

Is it a bug if it makes sense with the spec?

[Jakuje]

For PKCS#11 2.40, this is true, but it might be useful for the PKCS#11 3.0 and 3.2, where each of them defines new functions so if a caller will want to use some function from the 3.2 specs, but the underlying module will have only the 2.40 specs, this API would get useful, see #248 where the new functions are really supported only with the new function lists and we should get False for the old function lists.

I think implementing a test for this would be a good addition for the PR #248.