Add PBKDF2 support for gosthash94cp

Russian technical comitee working on standartization of cryptography
algorithms has published the document describing usage of GOST R
34.11-94 hash function with PBKDF2 algorithm (MR 26.2.001-2012).
Add test vectors from that document and a special function implementing
Nettle interface for PBKDF2 using gosthash94cp.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>