Main changes:
* 'system.hostname' config option is deprecated
* 'system.identity' is used for 'id.server' and 'hostname.bind'
* 'system.version' is used for 'version.server' and 'version.bind'
* In addition, config options (and NSID) support boolean on|off
* When on, the automatic default is filled on config load:
  * FQDN hostname (or at least hostname) for NSID and system.identity
  * Version string for system.version

The config options are still turned off by default.
refs #113

There was also a small error when a TCP master socket was removed,
the next one was skipped regardless of it's activity level.

If previous xfer socket was disconnected, the 'ret' variable wasn't cleared
when the current socket is inactive. The outcome of this is that the
innocent transfer could be discarded.

Thanks to Jonathan Hoppe from Netriplex for discovering this.

refs #111

* xfrin_add_new_node and ddns_add_new_node were duplicates
  - knot_node_set_zone() was called in the _add functions anyway
* merged checks for NSEC3-related RR to knot_rrset_is_nsec3rel()
  - the empty RR check was pointless, as even if empty, the node
    shouldn't be searched in normal tree

Refs #102

     - Inserting DNAMEs from out-of-zone RRs into tmp DNAME table messed
things up when we free'd the RRSet, it's better to check before
inserting into table (one could rollback the table, but there are some
corner cases not worth solving)

Refs #102

refs #100

Refs #102

- RFC 2136 states that the server should return the 'NOTZONE' error
  code, which it is correcly doing now, so there's no need to change it.
- Minor change only, since the actual check is somewhere before this code
  executes, but I've left the new_node functions as it was, since we
  may wish to allow some malformations later.

Refs #102

Refs #102

Refs #102

- Added an error handling function
- Changed some static functions a bit - they now return error codes
  instead of NULL

Refs #102

 - This out-of-zone check caused the whole transfer to fail. IXFR needs
   to be changed to, reverting to original master (I'd just delete the
master branch to rewind, but it is locked, but oh well)

Refs #102

- Server would refuse SOA records this way and transfers would fail.

Refs #102, fixes Coverity #1049750

Refs #102

refs #102