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)

 Index: binutils-2.19.51.0.5/ld/ld.texinfo

 ===================================================================

 diff -durN binutils-2.20.1.orig/ld/ld.texinfo binutils-2.20.1/ld/ld.texinfo

 --- binutils-2.20.1.orig/ld/ld.texinfo	2009-07-06 15:48:51.000000000 +0200

 +++ binutils-2.20.1/ld/ld.texinfo	2010-08-17 19:32:16.000000000 +0200

 @@ -2026,8 +2026,9 @@

  systems may not understand them. If you specify

  @option{--enable-new-dtags}, the dynamic tags will be created as needed.

  If you specify @option{--disable-new-dtags}, no new dynamic tags will be

 -created. By default, the new dynamic tags are not created. Note that

 -those options are only available for ELF systems.

 +created. On Gentoo, by default, the new dynamic tags are created (this

 +differs from upstream behaviour). Note that those options are only

 +available for ELF systems.

  @kindex --hash-size=@var{number}

  @item --hash-size=@var{number}