Refactor debug mode code

Refactor debug mode code

Knowledge of the debug mode snuck into the mdfour struct and code, which
did not feel good.

* Introduced a new “struct hash”, which is used by the hash_* functions
  instead of “struct mdfour”. “struct hash” contains the mdfour state
  and also debug file handles that can be set via a new
  hash_enable_debug function.
* Removed “identifier” from the mdfour struct and code.
* Introduced hash.h and all documented hash_* functions.
* Introduced unify.h.
* Removed some global state from util.c.