Fix hang in comsubs (rhbz#1062296) (re: 970069a6)

The new command substitution mechanism imported in 970069a6 from
Red Hat patches introduced this bug: backtick-style command
substitutions hang when processing about 117KiB of data or more.

It is fixed by another Red Hat patch:
642af4d6/f/ksh-20140415-hokaido.patch

It saves the value of the shp->comsub flag so that it is set to 2
(usually meaning new-style $(comsubs)) in two specific cases even
when processing backtick comsubs. This stops the sh_subtmpfile()
function in subshell.c from creating a /tmp file. However, I think
that approach is quite ugly, so I'm taking a slightly different one
that has the same effect.

src/cmd/ksh93/include/defs.h,
src/cmd/ksh93/sh/subshell.c:
- Redefine sh_subtmpfile() to pass the comsub flag as an argument.
  (Remove the shp pointer argument, which is redundant; a pointer
  to the shell state can easily be obtained in the function.)

src/cmd/ksh93/sh/xec.c: sh_exec():
- Apply the Red Hat fix by passing flag 2 to sh_subtmpfile().

src/cmd/ksh93/tests/subshell.sh:
- Move regress test from ce68e1be from basic.sh to here; this is
  the place for command substitution tests as they are subshells.
- Add regress test for this bug.

All other changed files:
- Update sh_subtmpfile() calls to pass on the shp->comsub flag.

src/cmd/ksh93/sh/subshell.c

@@ -106,10 +106,11 @@ static unsigned int subenv;
  * This routine will turn the sftmp() file into a real /tmp file or pipe
  * if the /tmp file create fails
  */
-void	sh_subtmpfile(Shell_t *shp)
+void	sh_subtmpfile(char comsub_flag)
 {
 	if(sfset(sfstdout,0,0)&SF_STRING)
 	{
+		Shell_t *shp = sh_getinterp();
 		register int fd;
 		register struct checkpt	*pp = (struct checkpt*)shp->jmplist;
 		register struct subshell *sp = subshell_data->pipe;
@@ -123,7 +124,7 @@ void	sh_subtmpfile(Shell_t *shp)
 		else if(errno!=EBADF)
 			errormsg(SH_DICT,ERROR_system(1),e_toomany);
 		/* popping a discipline forces a /tmp file create */
-		if(shp->comsub != 1)
+		if(comsub_flag != 1)
 			sfdisc(sfstdout,SF_POPDISC);
 		if((fd=sffileno(sfstdout))<0)
 		{
@@ -181,7 +182,7 @@ void sh_subfork(void)
 		trap = strdup(trap);
 	/* see whether inside $(...) */
 	if(sp->pipe)
-		sh_subtmpfile(shp);
+		sh_subtmpfile(shp->comsub);
 	shp->curenv = 0;
 	shp->savesig = -1;
 	if(pid = sh_fork(shp,FSHOWME,NIL(int*)))