xfstests: fsstress should kill children tasks before exit

It is very hard to predict runtime for fsstress. In many cases it
is useful to give test to run a reasonable time, and then kill it.
But currently there is no reliable way to kill test without leaving
running children.
This patch add sanity cleanup logic which looks follow:
 - On sigterm received by parent, it resend signal to it's children
 - Wait for each child to terminates
 - EXTRA_SANITY: Even if parent was killed by other signal, children
   will be terminated with SIGKILL to preven staled children.

So now one can simply run fsstress like this:
./fsstress -p 1000 -n999999999 -d $TEST_DIR &
PID=$!
sleep 300
kill $PID
wait $PID

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>

aclocal.m4

@@ -16,6 +16,11 @@ AC_DEFUN([AC_PACKAGE_WANT_LINUX_FIEMAP_H],
     AC_SUBST(have_fiemap)
   ])
 
+AC_DEFUN([AC_PACKAGE_WANT_LINUX_PRCTL_H],
+  [ AC_CHECK_HEADERS([sys/prctl.h], [ have_prctl=true ], [ have_prctl=false ])
+    AC_SUBST(have_prctl)
+  ])
+
 AC_DEFUN([AC_PACKAGE_WANT_FALLOCATE],
   [ AC_MSG_CHECKING([for fallocate])
     AC_TRY_LINK([