Add support for _POSIX2_VERSION, which lets you pick which POSIX

Add support for _POSIX2_VERSION, which lets you pick which POSIX
version you want the utilities to conform to.  Remove warnings about
failure to conform to a future POSIX version.

(usage): Document only the intersection of the
old and new behaviors, to encourage portability.
(main): Parse options using POSIX 1003.1-2001 rules if
conforming to that standard.  Do not warn of obsolete options.