Fix restoring DEBUG trap upon exiting virtual subshell

This trap failed to be restored correctly when being trapped in
a subshell, causing corruption or a crash when restoring the
parent shell environment's trap upon leaving the subshell.

Thanks to Koichi Nakashima for the report and reproducer.

src/cmd/ksh93/sh/fault.c: sh_sigreset():
- Fix an off-by-one error in the loop that restores the
  pseudosignal traps.

src/cmd/ksh93/tests/basic.sh:
- Test overwriting the main shell trap in a subshell for all
  pseudosignals.

Makes progress on: https://github.com/ksh93/ksh/issues/155

src/cmd/ksh93/sh/fault.c

@@ -344,7 +344,7 @@ void	sh_sigreset(register int mode)
 			sh.sigflag[sig] = flag;
 		}
 	}
-	for(sig=SH_DEBUGTRAP-1;sig>=0;sig--)
+	for(sig=SH_DEBUGTRAP; sig>=0; sig--)
 	{
 		if(trap=sh.st.trap[sig])
 		{