Prepare API for PBKDF that can set three costs
  - time (similar to iterations in PBKDF2)
  - memory (required memory for memory-hard function)
  - threads (required number of threads/CPUs).

This patch also removes wrongly designed API call
crypt_benchmark_kdf and replaces it with the new call
crypt_benchmark_pbkdf.

Two functions for PBKDF per context setting
are introduced: crypt_set_pbkdf_type and crypt_get_pbkdf_type.

The patch should be backward compatible when using
crypt_set_iteration_time function (works only for PBKDF2).

Signed-off-by: Milan Broz <gmazyland@gmail.com>