zdtm: rely on -D_GNU_SOURCE passed from Makefiles

After the commit
  02c763939c10 ("test/zdtm: unify common code")

CFLAGS with -D_GNU_SOURCE defined in the top Makefile
are being passed to tests Makefiles.
As _GNU_SOURCE is also defined by tests, that resulted in
zdtm tests build failures:

  make[2]: Entering directory `/home/criu/test/zdtm/lib'
   CC        test.o
  test.c:1:0: error: "_GNU_SOURCE" redefined [-Werror]
   #define _GNU_SOURCE
   ^
  <command-line>:0:0: note: this is the location of the previous definition
  cc1: all warnings being treated as errors
  make[2]: *** [test.o] Error 1

However, we didn't catch this in time by Travis-CI, as zdtm.py doesn't
do `make zdtm`, rather it does `make -C test/zdtm/{lib,static,transition}`.
By calling middle makefile this way, it doesn't have _GNU_SOURCE in
CFLAGS from top-Makefile.

I think the right thing to do here - is following CRIU's way:
rely on definition of _GNU_SOURCE by Makefiles.

This patch is almost fully generated with
  find test/zdtm/ -name '*.c' -type f					\
     -exec sed -i '/define _GNU_SOURCE/{n;/^$/d;}' '{}' \;		\
     -exec sed -i '/define _GNU_SOURCE/d' '{}' \;

With an exception for adding -D_GNU_SOURCE in tests Makefile.inc for
keeping the same behaviour for zdtm.py.
Also changed utsname.c to use utsname::domainname, rather private
utsname::__domainname, as now it's uncovered (from sys/utsname.h):
> struct utsname
>  {
...
> # ifdef __USE_GNU
>     char domainname[_UTSNAME_DOMAIN_LENGTH];
> # else
>     char __domainname[_UTSNAME_DOMAIN_LENGTH];
> # endif
Reported-by: Adrian Reber <areber@redhat.com>
Cc: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/static/uffd-events.c

-#define _GNU_SOURCE
 #include <errno.h>
 #include <fcntl.h>
 #include <unistd.h>