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)

 2008-06-07  Mike Frysinger  <vapier@gentoo.org>

 	* sysdeps/unix/sysv/linux/alpha/sys/user.h: Copy page.h definitions

 	from the kernel header to sys/user.h and remove the #include of

 	sys/user.h.

 --- a/sysdeps/unix/sysv/linux/alpha/sys/user.h

 +++ b/sysdeps/unix/sysv/linux/alpha/sys/user.h

 @@ -23,7 +23,6 @@

     only. Don't read too much into it. Don't use it for anything other

     than gdb/strace unless you know what you are doing. */

 -#include <asm/page.h>

  #include <asm/reg.h>

  struct user

 @@ -41,6 +40,9 @@ struct user

    char u_comm[32];				/* user command name */

  };

 +#define PAGE_SHIFT		13

 +#define PAGE_SIZE		(1 << PAGE_SHIFT)

 +#define PAGE_MASK		(~(PAGE_SIZE-1))

  #define NBPG			PAGE_SIZE

  #define UPAGES			1

  #define HOST_TEXT_START_ADDR	(u.start_code)