#16 Create functions to setting PBM parameters through CTX and CLI

[Martin Peylo]

The PBM parameters are hardcoded in the CTX to the most common ones, but they cannot yet be changed through a CTX function or the command line (at least OWF should also honor the -digest option).

hardcoded CTX parameters in cmp_ctx.c:

ctx->pbm_slen = 16;
ctx->pbm_owf = NID_sha256;
ctx->pbm_itercnt = 500;
ctx->pbm_mac = NID_hmac_sha1;