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)

Original patch from gentoo: gentoo/src/patchsets/gdb/6.8/20_all_gdb-tdep-opcode-include-workaround.patch

-= BEGIN original header =-

workaround build failure

http://bugs.gentoo.org/216368

-= END original header =-

diff -durN gdb-6.8.orig/gdb/frv-tdep.c gdb-6.8/gdb/frv-tdep.c

--- gdb-6.8.orig/gdb/frv-tdep.c	2008-01-11 14:19:59.000000000 +0100

+++ gdb-6.8/gdb/frv-tdep.c	2008-06-17 16:07:34.000000000 +0200

@@ -32,7 +32,7 @@

 #include "gdb_assert.h"

 #include "sim-regno.h"

 #include "gdb/sim-frv.h"

-#include "opcodes/frv-desc.h"	/* for the H_SPR_... enums */

+#include "../opcodes/frv-desc.h"	/* for the H_SPR_... enums */

 #include "symtab.h"

 #include "elf-bfd.h"

 #include "elf/frv.h"

diff -durN gdb-6.8.orig/gdb/mep-tdep.c gdb-6.8/gdb/mep-tdep.c

--- gdb-6.8.orig/gdb/mep-tdep.c	2008-01-11 14:20:02.000000000 +0100

+++ gdb-6.8/gdb/mep-tdep.c	2008-06-17 16:07:34.000000000 +0200

@@ -53,8 +53,8 @@

 /* Get the user's customized MeP coprocessor register names from

    libopcodes.  */

-#include "opcodes/mep-desc.h"

-#include "opcodes/mep-opc.h"

+#include "../opcodes/mep-desc.h"

+#include "../opcodes/mep-opc.h"

 /* The gdbarch_tdep structure.  */