Merge zone_t and zonedata_t

Merges public and private API for zones, resulting in new private API.

Functional tests are failing, the zone content switching may be broken:

18:03:57# KNOT TESTING SUITE 2014-02-03
18:03:57# Working directory /tmp/knottest-1391447037-obj4u7
18:03:57# Test 'axfr'
18:06:10#  * case 'bind_knot':  OK
18:06:32#  * case 'interrupt':  OK
18:08:16#  * case 'knot_bind':  OK
18:10:00#  * case 'knot_knot':  OK
18:10:00# Test 'basic'
18:10:08#  * case 'flags':      FAILED (IS SET TC flag)
18:10:16#  * case 'nsec':       FAILED (COMPARE sections AUTHORITY)
18:10:24#  * case 'nsec3':      FAILED (COMPARE sections AUTHORITY)
18:10:34#  * case 'query':      OK
18:10:34# Test 'chaos'
18:10:55#  * case 'ident':      OK
18:11:16#  * case 'version':    OK
18:11:16# Test 'ddns'
18:11:28#  * case 'forward':    FAILED (COMPARE UPDATE RCODE)
18:11:36#  * case 'prereq':     OK
18:11:36# Test 'dnssec'
18:11:55#  * case 'dnskey_timestamps':  FAILED (VALGRIND)
18:12:10#  * case 'none_to_nsec':       OK
18:12:24#  * case 'none_to_nsec3':      OK
18:13:25#  * case 'nsec3_to_nsec':      EXCEPTION (Can't query knot1 for siprouter. IN SOA.)
18:14:21#  * case 'nsec3_to_nsec_ddns': EXCEPTION (Can't get telnec61cce5. SOA>3138732158 from knot1.)
18:14:53#  * case 'nsec_to_nsec3':      OK
18:15:49#  * case 'nsec_to_nsec3_ddns': EXCEPTION (Can't get adserver. SOA>353732772 from knot1.)
18:15:50# Test 'edns'
18:16:16#  * case 'nsid':       OK
18:16:24#  * case 'payload':    OK
18:16:24# Test 'ixfr'
18:18:12#  * case 'from_differences':   FAILED (VALGRIND)
18:18:12# Test 'records'
18:18:25#  * case 'empty_rdata':        OK
18:18:33#  * case 'nsec_lettercase':    OK
18:18:41#  * case 'obsolete':   OK
18:18:48#  * case 'out-of-zone-master': OK
18:18:55#  * case 'out-of-zone-slave':  OK
18:18:55# Test 'security'
18:19:01#  * case 'dnsheader':  OK
18:19:01#  * case 'protos':     EXCEPTION ('c09-dns-query-r1.jar' PROTOS binary not found)
18:19:01# Test 'zone'
18:22:43#  * case 'load':       OK
18:22:43# TEST CASES: 30, FAILED: 10

@mvavrusa, your commits up to b4d78091 are reviewed and OK. Thanks for polishing this a little bit.

I checked the commits before my changes and all seems good or resolved, I'll try the final tests and merge this.