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)

 Respect environment CPPFLAGS when we run ./configure so we can inject

 random -D things without having to set CFLAGS/ASFLAGS

 diff -durN glibc-2.12.1.orig/Makeconfig glibc-2.12.1/Makeconfig

 --- glibc-2.12.1.orig/Makeconfig	2009-05-09 23:57:56.000000000 +0200

 +++ glibc-2.12.1/Makeconfig	2009-11-13 00:49:41.000000000 +0100

 @@ -697,6 +697,7 @@

  	   $(foreach lib,$(libof-$(basename $(@F))) \

  		         $(libof-$(<F)) $(libof-$(@F)),$(CPPFLAGS-$(lib))) \

  	   $(CPPFLAGS-$(<F)) $(CPPFLAGS-$(@F)) $(CPPFLAGS-$(basename $(@F)))

 +CPPFLAGS += $(CPPFLAGS-config)

  override CFLAGS	= -std=gnu99 $(gnu89-inline-CFLAGS) \

  		  $(filter-out %frame-pointer,$(+cflags)) $(+gccwarn-c) \

  		  $(sysdep-CFLAGS) $(CFLAGS-$(suffix $@)) $(CFLAGS-$(<F)) \

 diff -durN glibc-2.12.1.orig/config.make.in glibc-2.12.1/config.make.in

 --- glibc-2.12.1.orig/config.make.in	2009-05-09 17:59:38.000000000 +0200

 +++ glibc-2.12.1/config.make.in	2009-11-13 00:49:41.000000000 +0100

 @@ -107,6 +107,7 @@

  CXX = @CXX@

  BUILD_CC = @BUILD_CC@

  CFLAGS = @CFLAGS@

 +CPPFLAGS-config = @CPPFLAGS@

  ASFLAGS-config = @ASFLAGS_config@

  AR = @AR@

  RANLIB = @RANLIB@