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)

# D.U.M.A. - Detect Unintended Memory Access - Memory checker

## depends on ! BACKEND

## help D.U.M.A. - Detect Unintended Memory Access

## help A memory bound checker, with additional features.

## help Formerly known as Electric Fence.

config DUMA_A

    bool

    prompt "Build a static library"

    default y

config DUMA_SO

    bool

    prompt "Build a shared library"

    default y if SHARED_LIBS

choice

    bool

    prompt "D.U.M.A. version"

# Don't remove next line

# CT_INSERT_VERSION_BELOW

config DUMA_V_2_5_15

    bool

    prompt "2_5_15"

config DUMA_V_2_5_14

    bool

    prompt "2_5_14 (OBSOLETE)"

    depends on OBSOLETE

config DUMA_V_2_5_12

    bool

    prompt "2_5_12 (OBSOLETE)"

    depends on OBSOLETE

config DUMA_V_2_5_8

    bool

    prompt "2_5_8 (OBSOLETE)"

    depends on OBSOLETE

config DUMA_V_2_5_1

    bool

    prompt "2_5_1 (OBSOLETE)"

    depends on OBSOLETE

endchoice

config DUMA_VERSION

    string

# Don't remove next line

# CT_INSERT_VERSION_STRING_BELOW

    default "2_5_15" if DUMA_V_2_5_15

    default "2_5_14" if DUMA_V_2_5_14

    default "2_5_12" if DUMA_V_2_5_12

    default "2_5_8" if DUMA_V_2_5_8

    default "2_5_1" if DUMA_V_2_5_1