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)

 http://git.frugalware.org/gitweb/gitweb.cgi?p=frugalware-current.git;a=blob_plain;f=source/base/glibc/make-3.82-fix.patch;hb=8217c32ecc2e14962847ba3d8a272eb64a3dba4f

 --- glibc-2.10.1.OLD/manual/Makefile

 +++ glibc-2.10.1.NEW/manual/Makefile

 @@ -232,7 +232,9 @@

  .PHONY: stubs

  stubs: $(objpfx)stubs

  endif

 -$(objpfx)stubs ../po/manual.pot $(objpfx)stamp%:

 +$(objpfx)stubs ../po/manual.pot:

 +	touch $@

 +$(objpfx)stamp%:

  	$(make-target-directory)

  	touch $@