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)

 # Wrapper to build the debug facilities

 # List all debug facilities, and parse their scripts

 CT_DEBUG_FACILITY_LIST=

 for f in "${CT_LIB_DIR}/scripts/build/debug/"*.sh; do

     _f="$(basename "${f}" .sh)"

     _f="${_f#???-}"

     __f="CT_DEBUG_${_f}"

     if [ "${!__f}" = "y" ]; then

         CT_DoLog DEBUG "Enabling debug '${_f}'"

         . "${f}"

         CT_DEBUG_FACILITY_LIST="${CT_DEBUG_FACILITY_LIST} ${_f}"

     else

         CT_DoLog DEBUG "Disabling debug '${_f}'"

     fi

 done

 # Download the debug facilities

 do_debug_get() {

     for f in ${CT_DEBUG_FACILITY_LIST}; do

         do_debug_${f}_get

     done

 }

 # Extract and patch the debug facilities

 do_debug_extract() {

     for f in ${CT_DEBUG_FACILITY_LIST}; do

         do_debug_${f}_extract

     done

 }

 # Build the debug facilities

 do_debug() {

     for f in ${CT_DEBUG_FACILITY_LIST}; do

         do_debug_${f}_build

     done

 }