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)

http://sourceware.org/ml/libc-alpha/2002-10/msg00392.html

diff -durN glibc-2.10.1.orig/glibc-ports-2.10.1/sysdeps/mips/fpu_control.h glibc-2.10.1/glibc-ports-2.10.1/sysdeps/mips/fpu_control.h

--- glibc-2.10.1.orig/glibc-ports-2.10.1/sysdeps/mips/fpu_control.h	2009-05-16 10:36:20.000000000 +0200

+++ glibc-2.10.1/glibc-ports-2.10.1/sysdeps/mips/fpu_control.h	2009-11-13 00:51:31.000000000 +0100

@@ -86,7 +86,7 @@

 #define _FPU_RC_UP      0x2

 #define _FPU_RC_DOWN    0x3

-#define _FPU_RESERVED 0xfe3c0000  /* Reserved bits in cw */

+#define _FPU_RESERVED 0xfebc0000  /* Reserved bits in cw */

 /* The fdlibm code requires strict IEEE double precision arithmetic,

diff -durN glibc-2.10.1.orig/ports/sysdeps/mips/fpu_control.h glibc-2.10.1/ports/sysdeps/mips/fpu_control.h