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>

 diff -durN glibc-2.10.1.orig/localedata/locales/zh_TW glibc-2.10.1/localedata/locales/zh_TW

 --- glibc-2.10.1.orig/localedata/locales/zh_TW	2004-11-01 00:42:28.000000000 +0100

 +++ glibc-2.10.1/localedata/locales/zh_TW	2009-11-13 00:50:17.000000000 +0100

 @@ -1,7 +1,7 @@

  comment_char %

  escape_char /

  %

 -% Chinese language locale for Taiwan R.O.C.

 +% Chinese language locale for Taiwan

  % charmap: BIG5-CP950

  %

  % Original Author:

 @@ -17,7 +17,7 @@

  % Reference:	http://wwwold.dkuug.dk/JTC1/SC22/WG20/docs/n690.pdf

  LC_IDENTIFICATION

 -title      "Chinese locale for Taiwan R.O.C."

 +title      "Chinese locale for Taiwan"

  source     ""

  address    ""

  contact    ""

 @@ -25,7 +25,7 @@

  tel        ""

  fax        ""

  language   "Chinese"

 -territory  "Taiwan R.O.C."

 +territory  "Taiwan"

  revision   "0.2"

  date       "2000-08-02"

  %