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>

Original patch from gentoo: ../../dev/crosstool-NG/ct-ng.trunk/patches/strace/4.5.17/110-dont-use-REG_SYSCALL-for-sh.patch

-= BEGIN original header =-

diff -dur strace-4.5.16.orig/process.c strace-4.5.16/process.c

-= END original header =-

diff -durN strace-4.5.17.orig/process.c strace-4.5.17/process.c

--- strace-4.5.17.orig/process.c	2008-07-18 04:16:47.000000000 +0200

+++ strace-4.5.17/process.c	2008-10-24 21:30:13.000000000 +0200

@@ -2726,7 +2726,6 @@

        { 4*REG_GBR,            "4*REG_GBR"                             },

        { 4*REG_MACH,           "4*REG_MACH"                            },

        { 4*REG_MACL,           "4*REG_MACL"                            },

-       { 4*REG_SYSCALL,        "4*REG_SYSCALL"                         },

        { 4*REG_FPUL,           "4*REG_FPUL"                            },

        { 4*REG_FPREG0,         "4*REG_FPREG0"                          },

        { 4*(REG_FPREG0+1),     "4*REG_FPREG1"                          },