Checking DS digest length in XFR/DDNS.

- Functions for getting DS digest type and length from RDATA.
- Function for checking DS RDATA (compares the digest length with
  the expected length).
- Added constants for DS digest types and DNSSEC algorithm types
  (the latter is not used yet).
- Function for getting expected DS digest length.
- Modified adjusting functions to check the DS RDATA. Most of the
  function were modified to return some value.

refs #2050 @1h40m