Improve various things in the checksum code

Improve various things in the checksum code

- Size flist checksum data to hold the active size, not the max.
- Add a negotiated hash method to the daemon auth code.
- Use EVP for all openssl digests. This makes it easy to add more
  openssl digest methods and avoids deprecation warnings.
- Support a way to re-enable deprecated digests via openssl conf
  file and allow a default file to be configured.
- Supply a simple openssl-rsync.cnf file to enable legacy digests.