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)

 # Build script for strace

 do_debug_strace_get() {

     CT_GetFile "strace-${CT_STRACE_VERSION}" http://mesh.dl.sourceforge.net/sourceforge/strace/

     # Downloading from sourceforge leaves garbage, cleanup

     CT_DoExecLog ALL rm -f "${CT_TARBALLS_DIR}/showfiles.php"*

 }

 do_debug_strace_extract() {

     CT_Extract "strace-${CT_STRACE_VERSION}"

     CT_Patch "strace" "${CT_STRACE_VERSION}"

 }

 do_debug_strace_build() {

     CT_DoStep INFO "Installing strace"

     mkdir -p "${CT_BUILD_DIR}/build-strace"

     CT_Pushd "${CT_BUILD_DIR}/build-strace"

     CT_DoLog EXTRA "Configuring strace"

     CT_DoExecLog CFG                                        \

     "${CT_SRC_DIR}/strace-${CT_STRACE_VERSION}/configure"   \

         --build=${CT_BUILD}                                 \

         --host=${CT_TARGET}                                 \

         --prefix=/usr

     CT_DoLog EXTRA "Building strace"

     CT_DoExecLog ALL make

     CT_DoLog EXTRA "Installing strace"

     CT_DoExecLog ALL make DESTDIR="${CT_DEBUGROOT_DIR}" install

     CT_Popd

     CT_EndStep

 }