Pluggable libheimbase interface for DBs and misc libheimbase enhancements

[Code reviewed by Love Hörnquist Åstrand <lha@kth.se>]

    Added heim_db_*() entry points for dealing with databases, and
    make krb5_aname_to_localname() use it.

    The following enhancements to libheimbase are included:

     - Add heim_data_t and heim_string_t "reference" variants to
       avoid memory copies of potentially large data/strings.

       See heim_data_ref_create() and heim_string_ref_create().

     - Added enhancements to heim_array_t to allow their use for
       queues and stacks, and to improve performance.  See
       heim_array_insert_value().

     - Added XPath-like accessors for heim_object_t.  See
       heim_path_get(), heim_path_copy(), heim_path_create(), and
       heim_path_delete().  These are used extensively in the DB
       framework's generic composition of ACID support and in the
       test_base program

     - Made libheimbase more consistent with Core Foundation naming
       conventions.  See heim_{dict, array}_{get, copy}_value() and
       heim_path_{get, copy}().

     - Added functionality to and fixed bugs in base/json.c:
        - heim_serialize();
        - depth limit for JSON parsing (for DoS protection);
        - pretty-printing;
        - JSON compliance (see below);
        - flag options for parsing and serializing; these are needed
          because of impedance mismatches between heim_object_t and
          JSON (e.g., heim_dict_t allows non-string keys, but JSON
          does not; heimbase supports binary data, while JSON does
          not).

     - Added heim_error_enomem().

     - Enhanced the test_base program to test new functionality and
       to use heim_path*() to better test JSON encoding.  This
       includes some fuzz testing of JSON parsing, and running the
       test under valgrind.

     - Started to add doxygen documentation for libheimbase (but doc
       build for libheimbase is still incomplete).

    Note that there's still some incomplete JSON support:

     - JSON string quoting is not fully implemented;

     - libheimbase lacks support for real numbers, while JSON has
       it -- otherwise libheimbase is a superset of JSON,
       specifically in that any heim_object_t can be a key for an
       associative array.

    The following DB backends are supported natively:

     - "sorted-text", a binary search of sorted (in C locale), flat
       text files;

     - "json", a backend that stores DB contents serialized as JSON
       (this is intended for configuration-like contents).

    The DB framework supports:

     - multiple key/value tables per-DB
     - ACID transactions

    The DB framework also natively implements ACID transactions for
    any DB backends that a) do not provide transactions natively, b)
    do provide lock/unlock/sync methods (even on Windows).  This
    includes autocommit of DB updates outside transactions.

    Future DB enhancements may include:

     - add backends for various DB types (BDB, CDB, MDB, ...);

     - make libhdb use heim_db_t;

     - add a command-line tool for interfacing to databases via
       libheimbase (e.g., to get/set/delete values, create/copy/
       backup DBs, inspect history, check integrity);

     - framework-level transaction logging (with redo and undo
       logging), for generic incremental replication;

     - framework-level DB integrity checking.

       We could store a MAC of the XOR of a hash function applied to
       {key, value} for every entry in the DB, then use this to check
       DB integrity incrementally during incremental replication, as
       well as for the whole DB.

appl/dbutils/bsearch.c

@@ -140,13 +140,13 @@ main(int argc, char **argv)
     argc--;
     argv++;
 
-    ret = __bsearch_file_open(fname, max_size, block_size, &bfh, &reads);
+    ret = _bsearch_file_open(fname, max_size, block_size, &bfh, &reads);
     if (ret != 0) {
 	perror("bsearch_file_open");
 	return 1;
     }
 
-    __bsearch_file_info(bfh, &block_size, &max_size, &blockwise);
+    _bsearch_file_info(bfh, &block_size, &max_size, &blockwise);
     if (verbose_flag && blockwise) {
 	fprintf(stderr, "Using block-wise method with block size %lu and "
 		"cache size %lu\n",
@@ -172,11 +172,11 @@ main(int argc, char **argv)
 	    if (!*key)
 		continue;
 	}
-	ret = __bsearch_file(bfh, key, &value, &loc, &loops, &reads);
+	ret = _bsearch_file(bfh, key, &value, &loc, &loops, &reads);
 	if (ret != 0) {
 	    if (ret > 0) {
 		fprintf(stderr, "Error: %s\n", strerror(ret));
-		__bsearch_file_close(&bfh);
+		_bsearch_file_close(&bfh);
 		return 1;
 	    }
 	    if (verbose_flag)
@@ -200,6 +200,6 @@ main(int argc, char **argv)
     }
     if (failures)
 	return 2;
-    __bsearch_file_close(&bfh);
+    _bsearch_file_close(&bfh);
     return 0;
 }