Replaced all imports of test-skeleton.c with support/test-driver.c.

In some cases it was needed to adjust do_test to return int
instead of static int since that is the method's signature expected by
test-driver.c.

Checked on x86_64.

	* string/test-string.h (TEST_FUNCTION): Use test_main instead of
	test_main ().
	(CMDLINE_PROCESS): Use function instead of defined macro.
	* debug/test-strcpy_chk.c: Import support/test-driver.c and also
	<suppport/support.h> to use set_fortify_handler().
	* string/bug-envz1.c: Import support/test-driver.c instead of
	test-skeleton.c.
	* string/bug-strcoll2.c: Likewise.
	* string/bug-strtok1.c: Likewise.
	* string/stratcliff.c: Likewise.
	* string/test-ffs.c: Likewise.
	* string/test-memccpy.c: Likewise.
	* string/test-memchr.c: Likewise.
	* string/test-memcmp.c: Likewise.
	* string/test-memcpy.c: Likewise.
	* string/test-memmem.c: Likewise.
	* string/test-memmove.c: Likewise.
	* string/test-memrchr.c: Likewise.
	* string/test-memset.c: Likewise.
	* string/test-rawmemchr.c: Likewise.
	* string/test-strcasecmp.c: Likewise.
	* string/test-strcasestr.c: Likewise.
	* string/test-strcat.c: Likewise.
	* string/test-strchr.c: Likewise.
	* string/test-strcmp.c: Likewise.
	* string/test-strcpy.c: Likewise.
	* string/test-string.h: Likewise.
	* string/test-strlen.c: Likewise.
	* string/test-strncasecmp.c: Likewise.
	* string/test-strncat.c: Likewise.
	* string/test-strncmp.c: Likewise.
	* string/test-strncpy.c: Likewise.
	* string/test-strnlen.c: Likewise.
	* string/test-strpbrk.c: Likewise.
	* string/test-strrchr.c: Likewise.
	* string/test-strspn.c: Likewise.
	* string/test-strstr.c: Likewise.
	* string/tst-bswap.c: Likewise.
	* string/tst-cmp.c: Likewise.
	* string/tst-endian.c: Likewise.
	* string/tst-inlcall.c: Likewise.
	* string/tst-strcoll-overflow.c: Likewise.
	* string/tst-strfry.c: Likewise.
	* string/tst-strlen.c: Likewise.
	* string/tst-strtok.c: Likewise.
	* string/tst-strtok_r.c: Likewise.
	* string/tst-strxfrm.c: Likewise.
	* string/tst-strxfrm2.c: Likewise.
	* string/tst-svc.c: Likewise.
	* string/tst-svc2.c: Likewise.

Quite a few tests include libc-internal.h just for the DIAG_* macros.
Split those macros to their own file, which can be included safely in
_ISOMAC mode.  I also moved ignore_value, since it seems logically
related, even though I didn't notice any tests needing it.

Also add -Wnonnull suppressions to two tests that _should_ have them,
but the error is masked when compiling against internal headers.

	* include/libc-diag.h: New file.  Define ignore_value,
	DIAG_PUSH_NEEDS_COMMENT, DIAG_POP_NEEDS_COMMENT,
	DIAG_IGNORE_NEEDS_COMMENT, and DIAG_IGNORE_Os_NEEDS_COMMENT here.

	* include/libc-internal.h: Definitions of above macros moved from
	here.  Include libc-diag.h.  Add copyright notice.

	* malloc/tst-malloc.c, malloc/tst-memcheck.c, malloc/tst-realloc.c
	* misc/tst-error1.c, posix/tst-dir.c, stdio-common/bug21.c
	* stdio-common/scanf14.c, stdio-common/scanf4.c, stdio-common/scanf7.c
	* stdio-common/test-vfprintf.c, stdio-common/tst-printf.c
	* stdio-common/tst-printfsz.c, stdio-common/tst-sprintf.c
	* stdio-common/tst-unlockedio.c, stdio-common/tstdiomisc.c
	* stdlib/bug-getcontext.c, string/tester.c, string/tst-endian.c
	* time/tst-strptime2.c, wcsmbs/tst-wcstof.c:
	Include libc-diag.h instead of libc-internal.h.

	* stdlib/tst-environ.c: Include libc-diag.h.  Suppress -Wnonnull for
	call to unsetenv (NULL).
	* nptl/tst-mutex1.c: Include libc-diag.h.  Suppress -Wnonnull for
	call to pthread_mutexattr_destroy (NULL).

Building string/tst-endian.c with gcc 6 produces an build warning/error on s390 (big endian machine):
gcc tst-endian.c -c -std=gnu11 -fgnu89-inline  -O2 or -O3 ...
tst-endian.c: In function ‘do_test’:
tst-endian.c:16:30: error: self-comparison always evaluates to false [-Werror=tautological-compare]
    if (htobe16 (be16toh (i)) != i)
                              ^~
...

See definitions of htobexx, bexxtoh in string/endian.h:
...

This patch silences these warnings with DIAG_* macros if build with gcc 6
and newer.

The same warnings occur on little endian machines with the
"htoleXX (leXXtoh (i)) != i" if-statements.

ChangeLog:

	* string/tst-endian.c: Include <libc-internal.h>.
	(do_test): Ignore tautological-compare warnings around
	"htobeXX (beXXtoh (i)) != i" and
	"htoleXX (leXXtoh (i)) != i" if-statements.

	* string/endian.h: Add macros for fixed-size endian conversion.
	* bits/byteswap.h: Allow inclusion from <endian.h>.
	* sysdeps/i386/bits/byteswap.h: Likewise.
	* sysdeps/ia64/bits/byteswap.h: Likewise.
	* sysdeps/s390/bits/byteswap.h: Likewise.
	* sysdeps/x86_64/bits/byteswap.h: Likewise.
	* string/Makefile (tests): Add tst-endian.
	* string/tst-endian.c: New file.