Support __STDC_WANT_LIB_EXT2__ feature test macro.

This patch implements support for the __STDC_WANT_LIB_EXT2__ feature
test macro from ISO/IEC TR 24731-2:2010, thereby implementing one
possible approach for supporting ISO C feature test macros.

Recall that, as described in
<https://sourceware.org/ml/libc-alpha/2016-05/msg00486.html>, these
macros work based on the definition when affected headers are
included, so cannot be handled once when the first system header is
included because that might not be one of the headers the particular
macro in question affects.
<https://sourceware.org/ml/libc-alpha/2016-05/msg00680.html> expresses
views on possible approaches for implementation and
<https://sourceware.org/ml/libc-alpha/2016-06/msg00039.html> follows
up on that.

This patch arranges things so that the relevant condition is
__GLIBC_USE (LIB_EXT2), following one of the suggestions given.
Headers using these macros include <bits/libc-header-start.h>, which
in turn includes <features.h>.  Headers must define
__GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION before including
<bits/libc-header-start.h>, to discourage inclusion outside glibc as
requested.  __USE_GNU conditions on affected functions are changed to
__GLIBC_USE (LIB_EXT2), while it's added as an additional alternative
on the conditions for functions already enabled for some POSIX
versions.

It would be possible to convert existing __USE_* conditionals to
__GLIBC_USE (with the relevant __GLIBC_USE_* being defined in
<features.h> where __USE_* are presently defined), and so make them
typo-proof (given -Wundef -Werror in glibc builds) because __GLIBC_USE
is used with #if not #ifdef / #if defined.

No attempt is made to enforce the rule about diagnosing different
definitions of __STDC_WANT_LIB_EXT2__ when affected headers are
included; such a diagnostic is incompatible with multiple-include
guards on the affected headers, unless compiler extensions are added
to support it.

As previously noted, glibc does not implement all features from TR
24731-2:2010: the functions aswprintf vaswprintf getwdelim getwline
are not in glibc, although they would be appropriate to add if someone
wished to do so.  But I think it makes sense to support the feature
test macro if *any* of the controlled features are present in glibc.

Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by the patch).

	* bits/libc-header-start.h: New file.
	* Makefile (headers): Add bits/libc-header-start.h.
	* include/features.h (__STDC_WANT_LIB_EXT2__): Document.
	(__GLIBC_USE): New macro.
	* libio/stdio.h: Define
	__GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION and include
	<bits/libc-header-start.h> instead of including <features.h>.
	(fmemopen): Declare also if [__GLIBC_USE (LIB_EXT2)].
	(open_memstream): Likewise.
	(vasprintf): Declare if [__GLIBC_USE (LIB_EXT2)], not [__USE_GNU].
	(__asprintf): Likewise.
	(asprintf): Likewise.
	(__getdelim): Declare also if [__GLIBC_USE (LIB_EXT2)].
	(getdelim): Likewise.
	(getline): Likewise.
	* string/string.h: Define
	__GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION and include
	<bits/libc-header-start.h> instead of including <features.h>.
	(strdup): Declare also if [__GLIBC_USE (LIB_EXT2)]
	(strndup): Likewise.
	* wcsmbs/wchar.h: Define
	__GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION and include
	<bits/libc-header-start.h> instead of including <features.h>.
	(open_wmemstream): Declare also if [__GLIBC_USE (LIB_EXT2)].
	* manual/creature.texi (__STDC_WANT_LIB_EXT2__): Document macro.

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