specl: new --unicode command line option.

* specs/optparse_spec.yaml: Revert ASCIIfication of unicode
characters from earlier changeset.
* lib/specl.in: Add --unicode, and postamble clarifications to
OptionParser parameter.
* specs/specs.mk (SPECL_OPTS): Add --unicode option to support
specs/optparse_spec.yaml.
* lib/specl/loader.lua (parser_mt:refetch): Don't try to refetch
with incorrect indices when --unicode was passed.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>