Use C99-compliant scanf under _GNU_SOURCE with modern compilers.

Use C99-compliant scanf under _GNU_SOURCE with modern compilers.

The only difference between noncompliant and C99-compliant scanf is
that the former accepts the archaic GNU extension '%as' (also %aS and
%a[...]) meaning to allocate space for the input string with malloc.
This extension conflicts with C99's use of %a as a format _type_
meaning to read a floating-point number; POSIX.1-2008 standardized
equivalent functionality using the modifier letter 'm' instead (%ms,
%mS, %m[...]).

The extension was already disabled in most conformance modes:
specifically, any mode that doesn't involve _GNU_SOURCE and _does_
involve either strict conformance to C99 or loose conformance to both
C99 and POSIX.1-2001 would get the C99-compliant scanf.  With
compilers new enough to use -std=gnu11 instead of -std=gnu89, or
equivalent, that includes the default mode.

With this patch, we now provide C99-compliant scanf in all
configurations except when _GNU_SOURCE is defined *and*
__STDC_VERSION__ or __cplusplus (whichever is relevant) indicates
C89/C++98.  This leaves the old scanf available under e.g. -std=c89
-D_GNU_SOURCE, but removes it from e.g. -std=gnu11 -D_GNU_SOURCE (it
was already not present under -std=gnu11 without -D_GNU_SOURCE) and
from -std=gnu89 without -D_GNU_SOURCE.

There needs to be an internal override so we can compile the
noncompliant scanf itself.  This is the same problem we had when we
removed 'gets' from _GNU_SOURCE and it's dealt with the same way:
there's a new __GLIBC_USE symbol, DEPRECATED_SCANF, which defaults to
off under the appropriate conditions for external code, but can be
overridden by individual files within stdio.

We also run into problems with PLT bypass for internal uses of sscanf,
because libc_hidden_proto uses __REDIRECT and so does the logic in
stdio.h for choosing which implementation of scanf to use; __REDIRECT
isn't transitive, so include/stdio.h needs to bridge the gap with a
macro.  As far as I can tell, sscanf is the only function in this
family that's internally called by unrelated code.

Finally, there are several tests in stdio-common that use the
extension.  bug21.c is a regression test for a crash; it still
exercises the relevant code when changed to use %ms instead of %as.
scanf14.c through scanf17.c are more complicated since they are
actually testing the subtleties of the extension - under what
circumstances is 'a' treated as a modifier letter, etc.  I changed all
of them to use %ms instead of %as as well, but duplicated scanf14.c
and scanf16.c as scanf14a.c and scanf16a.c.  These still use %as and
are compiled with -std=gnu89 to access the old extension.  A bunch of
diagnostic overrides and manual workarounds for the old stdio.h
behavior become unnecessary.  Yay!

	* include/features.h (__GLIBC_USE_DEPRECATED_SCANF): New __GLIBC_USE
	parameter.  Only use deprecated scanf when __USE_GNU is defined
	and __STDC_VERSION__ is less than 199901L or __cplusplus is less
	than 201103L, whichever is relevant for the language being compiled.

	* libio/stdio.h, libio/bits/stdio-ldbl.h: Decide whether to redirect
	scanf, fscanf, sscanf, vscanf, vfscanf, and vsscanf to their
	__isoc99_ variants based only on __GLIBC_USE (DEPRECATED_SCANF).
	* wcsmbs/wchar.h: wcsmbs/bits/wchar-ldbl.h: Likewise for
	wscanf, fwscanf, swscanf, vwscanf, vfwscanf, and vswscanf.

	* libio/iovsscanf.c
	* libio/fwscanf.c
	* libio/iovswscanf.c
	* libio/swscanf.c
	* libio/vscanf.c
	* libio/vwscanf.c
	* libio/wscanf.c
	* stdio-common/fscanf.c
	* stdio-common/scanf.c
	* stdio-common/vfscanf.c
	* stdio-common/vfwscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-compat.c
	* sysdeps/ieee754/ldbl-opt/nldbl-fscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-fwscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-iovfscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-scanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-sscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-swscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-vfscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-vfwscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-vscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-vsscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-vswscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-vwscanf.c
	* sysdeps/ieee754/ldbl-opt/nldbl-wscanf.c:
	Override __GLIBC_USE_DEPRECATED_SCANF to 1.

	* stdio-common/sscanf.c: Likewise.  Remove ldbl_hidden_def for __sscanf.
	* stdio-common/isoc99_sscanf.c: Add libc_hidden_def for __isoc99_sscanf.
	* include/stdio.h: Provide libc_hidden_proto for __isoc99_sscanf,
	not sscanf.
	[!__GLIBC_USE (DEPRECATED_SCANF)]: Define sscanf as __isoc99_scanf
	with a preprocessor macro.

	* stdio-common/bug21.c, stdio-common/scanf14.c:
	Use %ms instead of %as, %mS instead of %aS, %m[] instead of %a[];
	remove DIAG_IGNORE_NEEDS_COMMENT for -Wformat.
	* stdio-common/scanf16.c: Likewise.  Add __attribute__ ((format (scanf)))
	to xscanf, xfscanf, xsscanf.

	* stdio-common/scanf14a.c: New copy of scanf14.c which still uses
	%as, %aS, %a[].  Remove DIAG_IGNORE_NEEDS_COMMENT for -Wformat.
	* stdio-common/scanf16a.c: New copy of scanf16.c which still uses
	%as, %aS, %a[].  Add __attribute__ ((format (scanf))) to xscanf,
	xfscanf, xsscanf.
	* stdio-common/scanf15.c, stdio-common/scanf17.c: No need to
	override feature selection macros or provide definitions of u_char etc.
	* stdio-common/Makefile (tests): Add scanf14a and scanf16a.
	(CFLAGS-scanf15.c, CFLAGS-scanf17.c): Remove.
	(CFLAGS-scanf14a.c, CFLAGS-scanf16a.c): New.  Compile these files
	with -std=gnu89.