Fix unsetting aliases in subshells

Aliases can now be correctly unset within subshell environments
(such as ( ... ), $(command substitutions), etc), as well as
non-subshell "shared" command substitutions (${ ...; }). Before,
attempts to unset aliases within these were silently ignored.

Prior discussion: https://github.com/att/ast/issues/108

Subshell alias trees are only referenced in a few places in the
code, *and* have always been broken, so this commit gets rid of the
whole notion of a subshell alias tree. Instead, there is now just
one flat alias tree, and subshells fork into a separate process
when aliases are set or unset within them. It is not really
conceivable that this could be a performance-sensitive operation,
or even a common one, so this is a clean fix with no downside.

src/cmd/ksh93/include/defs.h:
- Remove sh_subaliastree() definition.

src/cmd/ksh93/sh/subshell.c:
- Remove salias element (pointer to subshell alias tree) from
  subshell struct.
- Remove sh_subaliastree() function.
- sh_subshell(): Remove alias subshell tree cleanup.

src/cmd/ksh93/bltins/typeset.c:
- b_alias(): If in subshell, fork before setting alias.
- b_unalias(): If in subshell, fork before unsetting alias.
- unall(): Remove sh_subaliastree() call.

src/cmd/ksh93/sh/name.c:
- nv_open(): Remove sh_subaliastree() call.

src/cmd/ksh93/tests/subshell.sh:
- Add regression tests for unsetting or redefining aliases within
  subshells.

(cherry picked from commit 12a15605b9521a2564a6e657905705a060e89095)

src/cmd/ksh93/bltins/typeset.c

@@ -197,6 +197,8 @@ int    b_alias(int argc,register char *argv[],Shbltin_t *context)
 			troot = tdata.sh->track_tree;
 		}
 	}
+	if(context->shp->subshell && !context->shp->subshare)
+		sh_subfork();
 	return(setall(argv,flag,troot,&tdata));
 }
 
@@ -1142,6 +1144,8 @@ int    b_set(int argc,register char *argv[],Shbltin_t *context)
 int    b_unalias(int argc,register char *argv[],Shbltin_t *context)
 {
 	Shell_t *shp = context->shp;
+	if(shp->subshell && !shp->subshare)
+		sh_subfork();
 	return(unall(argc,argv,shp->alias_tree,shp));
 }
 
@@ -1161,11 +1165,7 @@ static int unall(int argc, char **argv, register Dt_t *troot, Shell_t* shp)
 	struct checkpt buff;
 	NOT_USED(argc);
 	if(troot==shp->alias_tree)
-	{
 		name = sh_optunalias;
-		if(shp->subshell)
-			troot = sh_subaliastree(0);
-	}
 	else
 		name = sh_optunset;
 	while(r = optget(argv,name)) switch(r)