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)

 --- libc/nptl/sysdeps/unix/sysv/linux/alpha/sysdep-cancel.h    2006/10/29 21:48:06     1.6

 +++ libc/nptl/sysdeps/unix/sysv/linux/alpha/sysdep-cancel.h    2007/03/13 21:28:03     1.7

 @@ -55,6 +55,7 @@

  	bne	a3, SYSCALL_ERROR_LABEL;			\

  __LABEL($pseudo_ret)						\

  	.subsection 2;						\

 +	cfi_startproc;						\

  __LABEL($pseudo_cancel)						\

  	subq	sp, 64, sp;					\

  	cfi_def_cfa_offset(64);					\

 @@ -90,12 +91,13 @@

  	cfi_def_cfa_offset(0);					\

  __LABEL($syscall_error)						\

  	SYSCALL_ERROR_HANDLER;					\

 +	cfi_endproc;						\

  	.previous

  # undef PSEUDO_END

  # define PSEUDO_END(sym)					\

 -	.subsection 2;						\

  	cfi_endproc;						\

 +	.subsection 2;						\

  	.size sym, .-sym

  # define SAVE_ARGS_0	/* Nothing.  */