Converted pgcrypto one too.

* Changed default randomness source to libc random()
  That way pgcrypto does not have any external dependencies
  and should work everywhere.
* Re-enabled pgcrypto build in contrib/makefile
* contrib/README update - there is more stuff than
  only 'hash functions'
* Noted the libc random fact in README.pgcrypto


Marko Kreen