scripts: fix dumping execution backtrace

Dumping the backtrace has been broken since changeset #652e56d6d35a:
scripts: execute each steps in a subshell

We can spawn sub-sub-shells in some cases.

The way the fault handler works is to dump the backtrace, but to avoid
printing it once for every sub-shell (which could get quite confusing),
it simply exits when it detects that it is being run in a sub-shell,
leaving to the top-level shell the work to dump the backtrace.

Because each step is executed in its own sub-shell, the variable arrays
that contain the step name, the source file and line number, are lost
when exiting the per-step sub-shell.

Hence, the backtrace is currently limited to printing only the top-level
main procedure of the shell.

Fix this thus:
- when dumping the bckatraces for the steps & the functions, remember
it was dumped, and only dump it if it was not already dumped
- at the top-level shell, print the hints

Also, rename the top-level step label.

Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

--- strace-orig/file.c	2008/05/20 04:56:18	1.92

+++ strace/file.c	2008/08/01 01:13:10	1.93

@@ -28,21 +28,22 @@

  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF

  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

  *

- *	$Id: file.c,v 1.92 2008/05/20 04:56:18 roland Exp $

+ *	$Id: file.c,v 1.93 2008/08/01 01:13:10 roland Exp $

  */

 #include "defs.h"

 #include <dirent.h>

+

 #ifdef LINUX

-#define dirent kernel_dirent

-#define dirent64 kernel_dirent64

-#include <linux/types.h>

-#include <linux/dirent.h>

-#undef dirent

-#undef dirent64

+struct kernel_dirent {

+	unsigned long   d_ino;

+	unsigned long   d_off;

+	unsigned short  d_reclen;

+	char            d_name[1];

+};

 #else

-#define kernel_dirent dirent

+# define kernel_dirent dirent

 #endif

 #ifdef LINUX