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>
(transplanted from 4193d6e6a17430a177fa88c287879c2c35e319f3)

diff -durN strace-4.5.19.orig/syscall.c strace-4.5.19/syscall.c

--- strace-4.5.19.orig/syscall.c	2009-10-28 18:47:18.000000000 +0100

+++ strace-4.5.19/syscall.c	2009-10-28 18:47:18.000000000 +0100

@@ -110,6 +110,18 @@

 #define TP TRACE_PROCESS

 #define TS TRACE_SIGNAL

+#ifndef HAVE_STATFS64

+/*

+ * Ugly hacks for systems that do not have LFS

+ */

+

+#define sys_truncate64  sys_truncate

+#define sys_ftruncate64 sys_ftruncate

+#define sys_getdents64  sys_getdents

+#define sys_statfs64    sys_statfs

+#define sys_fstatfs64   sys_fstatfs

+#endif

+

 static const struct sysent sysent0[] = {

 #include "syscallent.h"

 };