options-parser: provide a saner pluggable API.

options-parser: provide a saner pluggable API.

It's much too easy to forget that the functions you hook into
the option parser need to return unconsumed options in the
variable `func_run_hooks_result'; better to follow the
convention used in the rest of bootstrap and return results in a
variable named after the function with `_result' appended.
* libltdl/config/options-parser (func_run_hooks): implement this
new API.
(Option parsing): Update the example in the header comment for
this section to reflect the changes.
* bootstrap (bootstrap_options_prep, bootstrap_parse_options)
(bootstrap_validate_options): Adjust.
* bootstrap.conf (libtool_options_prep, libtool_parse_options)
(libtool_validate_options): Ditto.

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