Fork before entering shared-state command substitution

The code contains various checks to see if a subshell needs to
fork, like this one in the ulimit builtin:

	if(shp->subshell && !shp->subshare)
		sh_subfork();

All checks of this form are fatally broken, as each one of them
causes shared-state command substitutions to ignore parent virtual
subshells.

Currently the only feasible way to fix this is to fork a virtual
subshell before executing a shared-state command substitution in
it. In the long term I think shared-state command substitutions
should probably be redesigned to disassociate them completely from
the virtual subshell mechanism.

src/cmd/ksh93/sh/macro.c: comsubst():
- If we're in a non-subshare virtual subshell, fork it before
  entering a type 2 (subshare) command substitution.

src/cmd/ksh93/sh/subshell.c:
- sh_assignok(): Remove subshare fix from 911d6b06 as it's
  redundant now that the parent of a subshare is never a virtual
  subshell. Go back to not doing anything if the current "subshell"
  is a subshare.
- sh_subtracktree(), sh_subfuntree(): Similarly, remove the
  now-redundant subshare fixes from 13c57e4b.

src/cmd/ksh93/sh/xec.c: sh_exec():
- Fix a separate bug: only fork a virtual subshell before running a
  background job if that "subshell" is not a subshare.

src/cmd/ksh93/tests/subshell.sh:
- Add test for bug fixed in xec.c.
- Add tests for 'ulimit', 'builtin' and 'exec' run in subshare
  within subshell -- all commands that use checks of the form
  'if(sh.subshell && !sh.subshare) sh_subfork();'.

Resolves: https://github.com/ksh93/ksh/issues/289

src/cmd/ksh93/sh/macro.c

@@ -2075,6 +2075,7 @@ nosub:
 /*
  * This routine handles command substitution
  * <type> is 0 for older `...` version
+ * 1 for $(...) or 2 for ${ subshare; }
  */
 static void comsubst(Mac_t *mp,register Shnode_t* t, int type)
 {
@@ -2194,7 +2195,11 @@ static void comsubst(Mac_t *mp,register Shnode_t* t, int type)
 			type = 3;
 		}
 		else
+		{
+			if(type==2 && sh.subshell && !sh.subshare)
+				sh_subfork();	/* subshares within virtual subshells are broken, so fork first */
 			sp = sh_subshell(mp->shp,t,sh_isstate(SH_ERREXIT),type);
+		}
 		fcrestore(&save);
 	}
 	else