common: Add meta option ignore-invalid-option.

* common/argparse.c (iio_item_def_s, IIO_ITEM_DEF): New.
(initialize): Init field IIO_LIST.
(ignore_invalid_option_p): New.
(ignore_invalid_option_add): New.
(ignore_invalid_option_clear): New.
(optfile_parse): Implement meta option.

This option is currently of no use. However, as soon as it has been
deployed in all stable versions of GnuPG, it will allow the use of the
same configuration file with an old and a new version of GnuPG. For
example: If a new version implements the option "foobar", and a user
uses it in gpg.conf, an old version of gpg would bail out with the
error "invalid option". To avoid that the following line can be put
above that option in gpg.conf

ignore-invalid-option foobar

This meta option may be given several times or several option names
may be given as arguments (space delimited). Note that this option is
not available on the command line.