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)

 # This file declares functions to install the kernel headers for mingw

 # Copyright 2009 Bart vdr. Meulen

 # Licensed under the GPL v2. See COPYING in the root of this package

 CT_DoKernelTupleValues() {

     CT_TARGET_KERNEL="mingw32"

     CT_TARGET_SYS=

 }

 do_kernel_get() {

     CT_GetFile "w32api-${CT_W32API_VERSION}-mingw32-src" \

         http://downloads.sourceforge.net/sourceforge/mingw

 }

 do_kernel_extract() {

     CT_Extract "w32api-${CT_W32API_VERSION}-mingw32-src"

 }

 do_kernel_headers() {

     CT_DoStep INFO "Installing kernel headers"

     mkdir -p "${CT_HEADERS_DIR}"

     cp -r ${CT_SRC_DIR}/w32api-${CT_W32API_VERSION}-mingw32-src/include/*   \

           ${CT_HEADERS_DIR}

     CT_EndStep

 }