Allow adding CPE dict, lang_model or autodetect via oscap_source

And deprecated the old way via filenames.
Added error checking when adding CPEs in xccdf_session_load_cpe, use oscap_source

Also added tests testing the above.
Fixup the the error message.
Remove overabundant line

This is needed to keep ac_probes/configure.ac.tpl and configure.ac
in sync. The disparity has been brought in by
a827774ff5a3aa631c694af8a091493f6ecd10c1.
Introduce: xccdf_result_import_source.
trac#411: Remove references to docbook feature
Revert "trac#411: Remove references to docbook feature"

This reverts commit 8c4ab5e40eecac4e799fb525e6037ba24db387f7.