Don't persist redirs when failing to exec a program (re: 53937b9)

As of the referenced commit, the interactive shell no longer forces
an exit upon 'exec program' when 'program' cannot be executed. This
is good. However, any redirections to 'program' then persist in the
current interactive shell environment, which is wrong and harmful.
Further discussion: https://austingroupbugs.net/view.php?id=1896

src/cmd/ksh93/sh/xec.c: sh_exec(): case TCOM:
- Upon processing redirections for 'exec', when the shell is
  interactive (which excludes subshells of interactive shells) and
  at least one non-redirection operand was passed to 'exec', pass a
  type/flag argument of 0 to sh_redirect (in io.c) instead of 1;
  this makes sh_redirect save the FD state for later restoraton in
  case exec fails.
- Mildly refactor this whole code block for legibility. Get rid of
  opaque arithmetic with boolean 0/1 results.
- For normal command redirections, simplify the exec optimisation
  for redirections: remove checks for virtual subshell and EXIT
  trap after checking for nonzero execflg. These became redundant
  since execflg is set to 0 if check_exec_optimization returns
  false on lines 1108 and 1109. (re: b0a6c1b, 88aa369)

src/cmd/ksh93/sh/xec.c

@@ -1216,19 +1216,31 @@ int sh_exec(const Shnode_t *t, int flags)
 						if(io)
 						{
 							struct openlist *item;
+							/* Below, 'type' is the type flag for sh_redirect; see there for info */
 							if(np == SYSEXEC)		/* 'exec' */
-								type = 1 + !com[1];
-							else if(np == SYSREDIR)		/* 'redirect' */
-								if(!com[1])
-									type = 2;
+							{
+								/* Interactive shells no longer force an exit on failure to exec,
+								 * so if there is a program to 'exec', save the file descriptor
+								 * state (type==0) on interactive shells in case path_exec fails */
+								if(com[1])
+									type = sh_isstate(SH_INTERACTIVE) ? 0 : 1;
 								else
+									type = 2;	/* no operand: mark FD > 2 close-on-exec */
+							}
+							else if(np == SYSREDIR)		/* 'redirect' */
+							{
+								if(com[1])		/* do not allow operands */
 								{
 									errormsg(SH_DICT, ERROR_exit(2), "%s: %s: %s",
 										 SYSREDIR->nvname, e_badsyntax, com[1]);
 									UNREACHABLE();
 								}
+								type = 2;
+							}
+							else if(execflg)
+								type = 1;		/* don't bother to save redirection state */
 							else
-								type = (execflg && !sh.subshell && !sh.st.trapcom[0]);
+								type = 0;		/* normal non-persistent redirection */
 							sh.redir0 = 1;
 							sh_redirect(io,type);
 							for(item=buffp->olist;item;item=item->next)