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 make

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     CT_DoExecLog ALL chmod -R u+w "${CT_SRC_DIR}/make-${CT_MAKE_VERSION}"

     CT_Patch "make" "${CT_MAKE_VERSION}"

 }

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     CT_DoStep EXTRA "Installing make"

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

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     CT_DoExecLog CFG "${CT_SRC_DIR}/make-${CT_MAKE_VERSION}/configure" \

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     CT_DoExecLog ALL make

     CT_DoExecLog ALL make install

     CT_Popd

     CT_EndStep

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