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/tests/enum.sh

@@ -150,7 +150,7 @@ a=blue; let "a*=3" 2>/dev/null && err_exit "arithmetic can assign out of range (
 # Enum types should parse with 'command' prefix(es) and options and instantly
 # recognise subsequent builtins it creates, even as a oneliner, even with
 # shcomp. (This requires an ugly parser hack that this tests for.)
-got=$(eval 2>&1 'command command command enum -i -i -iii --igno -ii PARSER_t=(r g b); '\
+got=$(set +x; eval 2>&1 'command command command enum -i -i -iii --igno -ii PARSER_t=(r g b); '\
 'command command PARSER_t -r -rrAAA -A -rArArA -Arrrrrrr hack=([C]=G); typeset -p hack')
 exp='PARSER_t -r -A hack=([C]=g)'
 [[ $got == "$exp" ]] || err_exit "incorrect typeset output for enum with command prefix and options" \