-/* Describe how to deal with options that follow non-option ARGV-elements.
-
- If the caller did not specify anything,
- the default is REQUIRE_ORDER if the environment variable
- POSIXLY_CORRECT is defined, PERMUTE otherwise.
-
- REQUIRE_ORDER means don't recognize them as options;
- stop option processing when the first non-option is seen.
- This is what Unix does.
- This mode of operation is selected by either setting the environment
- variable POSIXLY_CORRECT, or using `+' as the first character
- of the list of option characters.
-
- PERMUTE is the default. We permute the contents of ARGV as we scan,
- so that eventually all the non-options are at the end. This allows options
- to be given in any order, even with programs that were not written to
- expect this.
-
- RETURN_IN_ORDER is an option available to programs that were written
- to expect options and other ARGV-elements in any order and that care about
- the ordering of the two. We describe each non-option ARGV-element
- as if it were the argument of an option with character code 1.
- Using `-' as the first character of the list of option characters
- selects this mode of operation.