Fix command substitutions in here-docs (rhbz#994241, rhbz#1036802)

When ksh was compiled with SHOPT_SPAWN (the default), any command
substitution embedded in a here-document returned an empty string.
The bug was also present in 93u+ 2012-08-01 (although not in every
case as some systems compile it without SHOPT_SPAWN).

This fixes it by applying a slightly edited combination of two Red
Hat patches (the second containing a fix for the first), which
backport a new command substitution mechanism from the abandoned
ksh 93v- beta version. The originals are:

642af4d6/f/ksh-20120801-macro.patch
642af4d6/f/ksh-20120801-fd2lost.patch

src/cmd/ksh93/include/io.h:
- The iopipe() function from xec.c is now needed in sh_subshell()
  (subshell.c), so rename it to sh_iounpipe() and declare it as an
  extern here. The 93v- beta did it as well. (The Red Hat patch did
  this without renaming it.)

src/cmd/ksh93/sh/xec.c:
- Backport new versions of iousepipe() and sh_iounpipe() from ksh
  93v-. New 'type' flaggery is introduced to distinguish between
  different command substitution conditions. What all that means
  remains to be determined.
- sh_exec(): I made one change to the Red Hat patch myself: if in a
  subshell and the type flags FAMP (for "ampersand" as in '&' as in
  background job) and TFORK are set, continue to call sh_subfork()
  to fork the subshell unconditionally, instead of only if we're in
  a command substitution connected to an unseekable file. Maybe the
  latter works for the 93v- code, but on 93u+(m) it causes a couple
  of regressions, which are fixed by my change:
  signal.sh[273]: subshell ignoring signal does not send signal to parent
  signal.sh[276]: subshell catching signal does not send signal to parent
  Details: https://github.com/ksh93/ksh/issues/104#issuecomment-696341902

src/cmd/ksh93/sh/macro.c,
src/cmd/ksh93/sh/subshell.c:
- Updates that go with those new functions.

Fixes:   https://github.com/ksh93/ksh/issues/104
Affects: https://github.com/ksh93/ksh/issues/124

src/cmd/ksh93/sh/macro.c

@@ -385,7 +385,7 @@ void sh_machere(Shell_t *shp,Sfio_t *infile, Sfio_t *outfile, char *string)
 				break;
 			    }
 			    case S_PAR:
-				comsubst(mp,(Shnode_t*)0,1);
+				comsubst(mp,(Shnode_t*)0,3);
 				break;
 			    case S_EOF:
 				if((c=fcfill()) > 0)
@@ -1150,7 +1150,7 @@ retry1:
 	    case S_PAR:
 		if(type)
 			goto nosub;
-		comsubst(mp,(Shnode_t*)0,1);
+		comsubst(mp,(Shnode_t*)0,3);
 		return(1);
 	    case S_DIG:
 		var = 0;