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)

 	Hack from Debian to hopefully get sandbox working on mips

 <`Kumba> ths: given the SHN_UNDEF thing is a hack, what's the preferred solution?

 <ths> For fakeroot the simplest trigger is "fakeroot file /bin/ls".

 <ths> `Kumba: I haven't found a better one yet.

 <ths> Probably marker symbols around the stub section, and then exclude it from the link map.

 <ths> This needs a ld change.

 diff -durN glibc-2.10.1.orig/elf/do-lookup.h glibc-2.10.1/elf/do-lookup.h

 --- glibc-2.10.1.orig/elf/do-lookup.h	2009-03-30 23:14:32.000000000 +0200

 +++ glibc-2.10.1/elf/do-lookup.h	2009-11-13 00:51:36.000000000 +0100

 @@ -248,6 +248,12 @@

  		}

  	      /* FALLTHROUGH */

  	    case STB_GLOBAL:

 +#ifdef __mips__

 +	      /* HACK: MIPS marks its lazy evaluation stubs with SHN_UNDEF

 +		 symbols, we skip them. */

 +	      if (sym->st_shndx == SHN_UNDEF)

 +		break;

 +#endif

  	      /* Global definition.  Just what we need.  */

  	      result->s = sym;

  	      result->m = (struct link_map *) map;