Add more support for POSIX 1003.1-2001, which requires removal

Add more support for POSIX 1003.1-2001, which requires removal
for support of obsolete "-DIGITS" option syntax in nice, and
which prohibits options with optional arguments like date's
-I option.

(ISO_8601_OPTION): New enum value.
(long_options): Use it.
(short_options): New constant.

(usage): Document the change.

(main): Conform to POSIX 1003.1-2001 if
POSIX2_VERSION says to, otherwise warn of obsolete usage if
OBSOLETE_OPTION_WARNINGS is nonzero and if not POSIXLY_CORRECT.