Introduction of oscap_source

This is pretty much an empty suit now.
Introduce very simple file openner to scap_source.
OVAL definition parser should use oscap_source.
OVAL result parser should use oscap_source.
OVAL variables parser should use oscap_source.
OVAL syschar parser should use oscap_source.
OVAL directives parser should use oscap_source.
CCE parser should use oscap_source
Refactor: Extract function: _oscap_determine_document_type_reader
Refactor: Move determine_type implementation to the source module tree.
Refactor: Extract function: _readable_origin.
Refactor: Extract function: _build_new_xmlTextReader
Introduce new responsibility for oscap_source: get_scap_type
Do not forward parser context to error handler.

The libxml_error_handler does not take any user data anyway.
oscap_source_get_xmlTextReader should ensure that error handler is set
_build_new_xmlTextReader could SetErrorHandler
Consolidate error messages from oscap_source

Once we have the code on the single place we can spot inconsistencies
easily.
Make oscap_source thingy public.
Refactor: Extract function: _oval_definition_model_merge_source
oval_definition_model_import should use oscap_source