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 83a09cd5c9ed9afd87a7d1d17319c2fd2203ad0f Mon Sep 17 00:00:00 2001

 From: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

 Date: Thu, 21 Jan 2010 10:36:13 +0100

 Subject: [PATCH 04/15] rpc: fix typo in version mismatch msg

 Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

 ---

  libc/inet/rpc/rpc_prot.c |    2 +-

  1 files changed, 1 insertions(+), 1 deletions(-)

 diff --git a/libc/inet/rpc/rpc_prot.c b/libc/inet/rpc/rpc_prot.c

 index 74658e6..229f988 100644

 --- a/libc/inet/rpc/rpc_prot.c

 +++ b/libc/inet/rpc/rpc_prot.c

 @@ -229,7 +229,7 @@ rejected (enum reject_stat rjct_stat,

  {

    switch (rjct_stat)

      {

 -    case RPC_VERSMISMATCH:

 +    case RPC_MISMATCH:

        error->re_status = RPC_VERSMISMATCH;

        return;

      case AUTH_ERROR:

 -- 

 1.6.6.1