Cache URI/PIN on objects when PIN is specified in URI

[simo5]

pkcs11 uris are used to search for objects.
A pin (or pin file) can be used as a pkcs11 URI parameter, in which case interactive pin requests are not needed.
Objects found using a URI provided PIN may later used in functions that require authentication (for example when CKA_ALWAYS_AUTHENTICATE is set on a key.
Caching the PIN on the key object would allow to easily provide the PIN to the Context Specific C_Login operation in those cases.

[simo5]

Related to #42

[Jakuje]

I have related issue that the pincache test is actually not testing caching of the PIN from UI prompt, but reads PIN from default openssl.cnf which contains the PIN. Trying with the openssl.cnf.nopin config, it correctly asks for PIN, but then fails in the child:

$ PRIURI="$ECPRI3URI" OPENSSL_CONF=tmp.softhsm/openssl.cnf.nopin ./pincache
Prompt: "Enter pass phrase for PKCS#11 Token (Slot 771260799 - SoftHSM slot ID 0x2df8817f):"
Returning: 12345678
Failed to EVP_DigestSignUpdate
405C49F35D7F0000:error:40800060:pkcs11:p11prov_sig_operate_init:The specified key handle is not valid:signature.c:791:Provided key has invalid handle

I am looking into this as part of #298 so I might be able to provide some fix in if I will notice something obvious.