Remove XPath usage from history initialisation code.

As a matter of fact, the index file of a history facility is very
straightforward which contains just a list of file abstract objects
for all existing raw history data files, therefore a simple for
loop suits best to traverse all items in the list. Whereas XPath
will be an overkill in this case.
Improve the initialisation code of history facility

The common for_each_file_name() is used to concise the logic to
establish history facilities for all existing folders under
<res>/histories/ folder on the harddrive.

Redundant code of sanity checks on the given directory name and traversal
of each subdir have been safely removed by using this common API.
Improve the error handling of obix_server_init

On error handling, if a facility has not been established successfully
in the first place relevant cleanup function should not be invoked,
although current code won't double dispose any facility.

Also renaming xmldb_dispose() to obix_xmldb_dispose() so as to be
compatible with naming conventions of other init/dispose functions