Use nettle for hashing when available rather than internal MD5/SHA1 routines

  Change the internal MD5/SHA1 routines to match nettle's name and
  calling convention and add suitable autoconf bits to auto-select
  nettle if it's available, otherwise fall back to the internal
  routines as usual.

  Not so much of an issue for MD5/SHA1 (though we might end up with
  more optimised routines in some instances), but allows easier use
  of other hashing/crypto functions in the future.