Functions for enabling GNU-style commpand-line option behavior.
gnu_styleAn object of class list of length 3.
Functions for enabling GNU-style command-line options. GNU-style options
are characterized by a single dash (-) before single character
option flags and a double dash (--) before multiple character
option flags.
By convention, gnu style options flags must begin with a letter; if numbers were allowed option flags would be ambiguous with negative option values.
[GNU Command Line Standards]http://www.gnu.org/prep/standards/standards.html
Non-exported function *_flag_test, *_flag_to_name and
*_name_to_flag