core: Add features for pretty printing the help

* src/gpg-error.h.in (ARGPARSE_OPT_HEADER): New.
(ARGPARSE_OPT_VERBATIM): New.
(ARGPARSE_verbatim): New.
(ARGPARSE_header): New.
* src/argparse.c (show_help): Implement them.

These two macros are useful for custom help screens. In contrast to
the ARGPARSE_group hack they do not need a short option number (we use
1 here as a surrogate) and not the "@" hack. Thus already translated
strings can be used. ARGPARSE_header further allows to specify a
symbolic name for the group and does some minor formatting. If such a
symbolic name is given --dump-option-table also emits this as a pseudo
option for consumption by GUIs which for example use tabs to group
options.

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