Allow marking options as ignored.

* jnlib/argparse.h (ARGPARSE_OPT_IGNORE): New.
(ARGPARSE_TYPE_MASK): New, for internal use.
(ARGPARSE_ignore): New.
* jnlib/argparse.c (optfile_parse, arg_parse): Replace remaining
constants by macros.
(optfile_parse): Implement ARGPARSE_OPT_IGNORE.
(arg_parse): Exclide ignore options from --dump-options.

In addition to the ignore-invalid-option (commit 8ea49cf5) it is often
useful to mark options in a configuration which as NOP. For example
options which have no more function at all but can be expected to be
found in existing conf files. Such an option (or command) may now be
given as

ARGPARSE_ignore (300, "obsolete-option")

The 300 is merely used as a non-valid single option name much like
group names or the 500+n values used for long options.

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