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)

 Original patch from Gentoo.

 -= BEGIN original header =-

 -= END original header =-

 diff -durN uClibc-0.9.30.orig/Rules.mak uClibc-0.9.30/Rules.mak

 --- uClibc-0.9.30.orig/Rules.mak	2008-11-12 13:24:16.000000000 +0100

 +++ uClibc-0.9.30/Rules.mak	2009-02-07 09:57:59.000000000 +0100

 @@ -519,6 +519,7 @@

  endif

  CFLAGS += $(call check_gcc,-std=gnu99,)

 +CFLAGS += $(call check_gcc,-fgnu89-inline,)

  LDFLAGS_NOSTRIP:=$(CPU_LDFLAGS-y) -shared \

  	-Wl,--warn-common -Wl,--warn-once -Wl,-z,combreloc