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)

 From: Tomoyoshi ASANO <asa@lineo.co.jp>

 Message ID: <20081208073522.0AEF.22C34B8C@lineo.co.jp>

 Hello,

 I have SEGV at busybox's telnet with uClibc-0.9.30.

 It seem the current uClibc-trunk has the same problem.

 Thanks,

 -- Tom

 diff -ru uClibc-0.9.30.orig/libc/inet/getaddrinfo.c uClibc-0.9.30/libc/inet/getaddrinfo.c

 --- uClibc-0.9.30.orig/libc/inet/getaddrinfo.c	2008-10-28 17:25:10.000000000 +0100

 +++ uClibc-0.9.30/libc/inet/getaddrinfo.c	2009-02-26 14:20:06.000000000 +0100

 @@ -186,7 +186,7 @@

  			return seen;

  		}

 -		for (runp = ifa; runp != NULL; runp = runp->ifa_next)

 +		for (runp = ifa; runp != NULL; runp = runp->ifa_next) {

  #if defined __UCLIBC_HAS_IPV4__

  			if (runp->ifa_addr->sa_family == PF_INET)

  				seen |= SEEN_IPV4;

 @@ -195,7 +195,7 @@

  			if (runp->ifa_addr->sa_family == PF_INET6)

  				seen |= SEEN_IPV6;

  #endif /* __UCLIBC_HAS_IPV6__ */

 -

 +                }

  		freeifaddrs(ifa);

  	}

  #else