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)

--- uClibc-0.9.30.orig/extra/scripts/unifdef.c	2008-04-28 01:10:00.000000000 +0200

+++ uClibc-0.9.30/extra/scripts/unifdef.c	2011-08-21 09:42:21.000000000 +0200

@@ -206,7 +206,7 @@

 static void             error(const char *);

 static int              findsym(const char *);

 static void             flushline(bool);

-static Linetype         getline(void);

+static Linetype         getlinetype(void);

 static Linetype         ifeval(const char **);

 static void             ignoreoff(void);

 static void             ignoreon(void);

@@ -512,7 +512,7 @@

 	for (;;) {

 		linenum++;

-		lineval = getline();

+		lineval = getlinetype();

 		trans_table[ifstate[depth]][lineval]();

 		debug("process %s -> %s depth %d",

 		    linetype_name[lineval],

@@ -526,7 +526,7 @@

  * help from skipcomment().

  */

 static Linetype

-getline(void)

+getlinetype(void)

 {

 	const char *cp;

 	int cursym;