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
```