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)

 # PPL options

 choice

     bool

     prompt "PPL version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config PPL_V_0_11_2

     bool

     prompt "0.11.2 (EXPERIMENTAL)"

     depends on EXPERIMENTAL

     select PPL_0_11

 config PPL_V_0_11_1

     bool

     prompt "0.11.1 (EXPERIMENTAL)"

     depends on EXPERIMENTAL

     select PPL_0_11

 config PPL_V_0_11

     bool

     prompt "0.11 (EXPERIMENTAL)"

     depends on EXPERIMENTAL

     select PPL_0_11

 config PPL_V_0_10_2

     bool

     prompt "0.10.2"

 endchoice

 config PPL_VERSION

     string

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "0.11.2" if PPL_V_0_11_2

     default "0.11.1" if PPL_V_0_11_1

     default "0.11" if PPL_V_0_11

     default "0.10.2" if PPL_V_0_10_2

 # For PPL 0.11, we need to pull libpwl if configured static

 config PPL_0_11

     bool

     select PPL_NEEDS_LIBPWL if ! COMPLIBS_SHARED

 config PPL_NEEDS_LIBPWL

     bool