Some improvements for TSIG handling in PowerDNS

- Fixed bugs in TSIG key handling
- Now supports MD5 and SHA1/SHA224/SHA256/SHA384/SHA512 algorithms
- New TSIG key management API and implementation in some backends
- pdnssec now supports TSIG key management
- Test suite upgrades for all this
- Documentation for new pdnssec commands
- Multiple fixes and improvements to regression test suite