clean up rbtdb.c

in preparation for splitting up rbtdb.c, rename some types so they
can be defined in dns/types.h instead of only locally. these include:

- struct noqname, which is used to hold no-qname and closest-encloser
  proofs, and is now named dns_proof_t;
- rbtdb_rdatatype_t, which is used to hold a pair of rdatatypes and
  is now called dns_typepair_t and defined in rdatatype.h;
- rbtdb_serial_t, which is now just a uint32_t;
- rdatasetheader_t and rdatasetheaderlist_t, now called
  dns_slabheader_t and dns_slabheaderlist_t;
- rbtdb_version_t, now called dns_rbtdb_version_t.

the helper functions header_from_raw() and raw_from_header() are
renamed dns_slabheader_fromrdataset() and dns_slabheader_raw().

also made further style changes:
- fixing uninitialized pointer variables throughout rbtdb.c;
- switching some initializations to struct literals;
- renaming some functions and struct members more descriptively;
- replacing dns_db_secure_t with a simple bool since it no longer needs
  to be tri-valued.

tests/dns/rbtdb_test.c

@@ -98,7 +98,7 @@ ownercase_test_one(const char *str1, const char *str2) {
 	rbtdb_nodelock_t node_locks[1];
 	dns_rbtdb_t rbtdb = { .node_locks = node_locks };
 	dns_rbtnode_t rbtnode = { .locknum = 0 };
-	rdatasetheader_t header = { 0 };
+	dns_slabheader_t header = { 0 };
 	unsigned char *raw = (unsigned char *)(&header) + sizeof(header);
 	dns_rdataset_t rdataset = {
 		.magic = DNS_RDATASET_MAGIC,
@@ -158,7 +158,7 @@ ISC_RUN_TEST_IMPL(setownercase) {
 	rbtdb_nodelock_t node_locks[1];
 	dns_rbtdb_t rbtdb = { .node_locks = node_locks };
 	dns_rbtnode_t rbtnode = { .locknum = 0 };
-	rdatasetheader_t header = { 0 };
+	dns_slabheader_t header = { 0 };
 	unsigned char *raw = (unsigned char *)(&header) + sizeof(header);
 	dns_rdataset_t rdataset = {
 		.magic = DNS_RDATASET_MAGIC,