gpgconf: Fix listing of default_pubkey_algo.

* tools/gpgconf-comp.c (known_options_gpg, known_options_gpgsm): No
flags needed for pseudo options.
(known_pseudo_options_gpg, known_pseudo_options_gpgsm): New.
(gc_component): Add field known_pseudo_options.
(struct read_line_wrapper_parm_s): New.
(read_line_wrapper): New.
(retrieve_options_from_program): Use read_line_wrapper to handle
pseudo options.

Due to reading the list of options from the argparser of the
component, we had no more information about the pseudo options and
thus could not emit them. Well, there is just one pseudo option right
now, but with this change we have a generalized concept for them:
Pseudo options are used to convey information from a component to
gpgconf; for example build-in values.

• GnuPG-bug-id: T5341