threaded-am: better serialization for required config files

threaded-am: better serialization for required config files

With this change, we make serialization/de-serialization of
required config files installs more granular, and in the process
fix the bug introduced by commit `v1.11-1219-g326ecba'.

* automake.in ($required_conf_file_queue): Move its declaration
earlier.
(require_file_internal): Add a new argument telling whether the
function should act immediately or queue its action for the master
thread to handle.
(queue_required_conf_file): Renamed ...
(queue_required_file_check_or_copy): ... to this.
(require_queued_conf_file): Renamed ...
(require_queued_file_check_or_copy): ... to this, and make it call
`required_file_check_or_copy' instead of `require_file_internal'.
(require_conf_file, handle_makefiles_threaded): Adjust and simplify
accordingly.
* tests/Makefile.am (XFAIL_TESTS): Remove `parallel-am.test'.