getopt-gnu: match recent glibc fixes and posix ruling

getopt-gnu: match recent glibc fixes and posix ruling

The POSIX folks admitted that codifying the behavior of GNU
getopt on a leading '+' in optstring is worthwhile, for writing
programs such as env(1) even when POSIXLY_CORRECT is not defined.
http://austingroupbugs.net/view.php?id=191
However, the ruling is an enhancement request for the next
version of POSIX, and is not binding on platforms that comply
with POSIX 2008, so it should only be enforced for getopt-gnu.

* tests/test-getopt.h (test_getopt): Strengthen tests of leading
'+' handling, when requesting extensions.
* tests/test-getopt_long.h (test_getopt_long): Strengthen test of
'W;' handling.
* m4/getopt.m4 (gl_GETOPT_CHECK_HEADERS): Detect glibc 2.11 bug.
* doc/posix-functions/getopt.texi (getopt): Document this.
* doc/glibc-functions/getopt_long.texi (getopt_long): Likewise.
* doc/glibc-functions/getopt_long_only.texi (getopt_long_only):
Likewise.