gpgconf: Fix --list-options for forced options

* tools/gpgconf-comp.c (retrieve_options_from_program): Ignore to to
ignored options.  Add failsafe code to avoid calling percent_escape
with NULL.

Remember: When using ARGPARSE_FLAG_WITHATTR the ARGPARSE_OPT_IGNORE
flags in the returned type needs to be considered.

• GnuPG-bug-id: T5800