stdnoreturn, stdnoreturn-tests: remove modules

stdnoreturn, stdnoreturn-tests: remove modules

They're not needed here and a bit premature for use elsewhere.  See
<http://lists.gnu.org/archive/html/bug-gnulib/2011-07/msg00209.html>.
* m4/stdnoreturn.m4, modules/stdnoreturn, modules/stdnoreturn-tests:
* tests/test-stdnoreturn.c: Remove files.
* MODULES.html.sh (_Noreturn <stdnoreturn.h>): Remove section.
* lib/openat.h, lib/sigpipe-die.h, lib/xalloc.h, lib/xmemdup0.h:
* lib/xstrtol.h: Use _Noreturn rather than including <stdnoreturn.h>
and using noreturn.
* modules/openat, modules/sigpipe-die, modules/xalloc:
* modules/xmemdup0, modules/xstrtol:
Remove dependency on stdnoreturn.