gpgconf: Support reading global options (part 1).

* tools/gpgconf.c (main): Set the coinfig directories.
* tools/gpgconf-comp.c (gc_backend): Change the name of the config
files.
(struct gc_option): Add new field 'attr'.
(retrieve_options_from_program): Rewrite to use gpgrt_argparser.

We need to do larger changes to gpgconf so that it is possible to get
also global config options and their attributes. The old code worked
along its own list of option and used a generic option file parser.
This has no support for global config files. We now use
gpgrt_argparser so that we to do exactly the same as the component
does and thus delivers the actual option values as seen by the
component.

This is just a first step and we need to change more things.

• Signed-off-by: Werner Koch <wk@gnupg.org>