Turn off SH_INTERACTIVE state flag in subshells

By definition, subshells are never interactive, so they should
disable behaviour associated with interactive shells even if the
main shell is interactive.

Most visibly, running a background job from a subshell like
	( some_command & )
now no longer prints a job ID that you cannot use in the main shell.
This behaviour change matches pdksh/mksh, bash, zsh, dash, et al.

Prior discussion:
https://www.mail-archive.com/austin-group-l@opengroup.org/msg06456.html
(plus the preceding thread)

src/cmd/ksh93/sh/subshell.c: sh_subshell():
- Before running the command(s) in the subshell using sh_exec(),
  turn off the SH_INTERACTIVE shell state flag. (No need to add
  code to restore it as this function already saves and restores
  the entire shell state.)

src/cmd/ksh93/bltins/misc.c: b_bg():
- If there is no job control when using 'bg', 'fg' or 'disown',
  always print the "no job control" error message and not only if
  the shell is in the interactive state. This is also what
  pdksh/mksh, bash and zsh do.

src/cmd/ksh93/bltins/misc.c

@@ -418,8 +418,7 @@ int    b_bg(register int n,register char *argv[],Shbltin_t *context)
 	argv += opt_info.index;
 	if(!sh_isoption(SH_MONITOR) || !job.jobcontrol)
 	{
-		if(sh_isstate(SH_INTERACTIVE))
-			errormsg(SH_DICT,ERROR_exit(1),e_no_jctl);
+		errormsg(SH_DICT,ERROR_exit(1),e_no_jctl);
 		return(1);
 	}
 	if(flag=='d' && *argv==0)

src/cmd/ksh93/sh/subshell.c

@@ -633,6 +633,7 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 		if(shp->savesig < 0)
 		{
 			shp->savesig = 0;
+			sh_offstate(SH_INTERACTIVE);
 			sh_exec(t,flags);
 		}
 	}