Fix crash in xtrace while processing here-document (re: d7cada7b)

Depending on the OS, the heredoc.sh regression tests, and possibly
others, still crashed with the -x option (xtrace) on.

Analysis: The lexer crashes in lex_advance(). Something has caused
an inconsistent lexer state, and it happened earlier on, so the
backtrace is useless for figuring out where that happened.

But I think I've found it. It's the sh_mactry() call here:

src/cmd/ksh93/sh/xec.c, lines 2800 to 2807 in f7213f03
2800:   if(!(cp=nv_getval(sh_scoped(shp,PS4NOD))))
2801:           cp = "+ ";
2802:   else
2803:   {
2804:           sh_offoption(SH_XTRACE);
2805:           cp = sh_mactry(shp,cp);
2806:           sh_onoption(SH_XTRACE);
2807:   }

sh_mactry() needs to parse the contents of $PS4 to perform
expansions and command substitutions in it, which involves the
lexer. If that happens in a here-document, the lexer is in the C
function call stack, in the middle of parsing the here-document.
Result: inconsistent lexer state. Solution: save and restore lexer
state in sh_mactry().

After this commit, all regression tests should pass with the
'-x'/'--xtrace' option in use, with no errors or crashes.

Note for backporters: this fix depends both on on d7cada7b and on
the consistency fix for the Lex_t type's size applied in a7ed5d9f.

src/cmd/ksh93/include/shlex.h:
- Cosmetic fix: remove a copied & pasted backslash. (re: a7ed5d9f)

src/cmd/ksh93/sh/macro.c: sh_mactry():
- Save and restore the lexer state before letting sh_mactrim()
  indirectly parse and execute code.

src/cmd/ksh93/tests/*.sh:
- Turn off xtrace in various command substitutions that contain
  2>&1 redirections, so that the xtrace output is not caught by
  the command substitutions, causing tests to fail incorrectly.
- Turn off xtrace for a few code blocks with 2>&1 redirections,
  stopping xtrace output from being written to standard output.

Resolves: https://github.com/ksh93/ksh/issues/306 (again)

src/cmd/ksh93/sh/macro.c

@@ -124,6 +124,7 @@ void *sh_macopen(Shell_t *shp)
 
 /*
  * perform only parameter substitution and catch failures
+ * (also save lexer state to allow use while in here-docs)
  */
 char *sh_mactry(Shell_t *shp,register char *string)
 {
@@ -132,11 +133,13 @@ char *sh_mactry(Shell_t *shp,register char *string)
 		int		jmp_val;
 		int		savexit = shp->savexit;
 		struct checkpt	buff;
+		Lex_t		*lexp = (Lex_t*)sh.lex_context, savelex = *lexp;
 		sh_pushcontext(shp,&buff,SH_JMPSUB);
 		jmp_val = sigsetjmp(buff.buff,0);
 		if(jmp_val == 0)
 			string = sh_mactrim(shp,string,0);
 		sh_popcontext(shp,&buff);
+		*lexp = savelex;
 		shp->savexit = savexit;
 		return(string);
 	}