gpgconf: New command --show-configs.

* tools/gpgconf.c (aShowConfigs): New.
(opts): Add --show-configs.
(CUTLINE_FMT): New.
(show_version_gnupg): Add arg "prefix" and adjust caller.
(my_copy_file): New.
(show_configs_one_file): New.New.
(show_configs): New.
(main): Call show_configs.

The ability to have a consolidated list of all config files is very
useful for support cases. This is in particular important due to the
global config files and their conditional constructs.

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