gpgconf: Make WoT settings configurable by gpgconf.

* tools/gpgconf-comp.c (gc_options_gpg): Add max-cert-depth,
completes-needed, and marginals-needed options.
* g10/gpg.c (gpgconf_list): Likewise.

Some tests to come for the PGP trust model will need to manipulate
these parameters.

• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>