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)

 In memoriam: Cédric DUVAL.

 This release is dedicated to my good friend, Cédric, who passed

 away in late September 2011, after a long and stressful struggle

 against the disease.

 Cédric was always prompt to discuss any subject. He was open-minded,

 and every thing on Earth was of interest to him; Free Software, and

 Freedom in general, was one of those interests we used to discuss

 about most. He and I had many a good talk about so many things, and

 I learned so much from him.

 Cédric leaves behind him loving and dismayed girl friend and father

 and mother, and countless saddened friends; we will all dearly miss

 him.

 Do not forget your towel, Cédric.

 So long, and thanks for all the fish!