Use internal PBKDF2 implementation for OpenBSD softraid hashing

As recommended by magnum.

Before:
```
Benchmarking: openbsd-softraid, OpenBSD SoftRAID (8192 iterations) [PBKDF2-SHA1 32/64]... (4xOMP) DONE
Raw:    80.7 c/s real, 21.1 c/s virtual
```
After:
````
Benchmarking: openbsd-softraid, OpenBSD SoftRAID (8192 iterations) [PBKDF2-SHA1 32/64]... (4xOMP) DONE
Raw:    1056 c/s real, 275 c/s virtual
```

@tweksteen It would be awesome if we can use our own HMAC code instead of using OpenSSL's HMAC() function (due to allocations and potential locking issues in the OpenSSL code).