Fix redefining & unsetting functions in subshells (BUG_FNSUBSH)

Functions can now be correctly redefined and unset in subshell
environments (such as ( ... ), $(command substitutions), etc).
Before this fix, attempts to do this were silently ignored (!!!),
causing the wrong code (i.e.: the function by the same name from
the parent shell environment) to be executed.

Redefining and unsetting functions within "shared" command
substitutions of the form '${ ...; }' is also fixed.

Prior discussion: https://github.com/att/ast/issues/73

src/cmd/ksh93/sh/parse.c:
- A fix from George Koelher (URL above). He writes:
  | The parser can set t->comnamp to the wrong function.
  | Suppose that the shell has executed
  |     foo() { echo WRONG; }
  | and is now parsing
  |     (foo() { echo ok; } && foo)
  | The parser was setting t->comnamp to the wrong foo. [This
  | fix] doesn't set t->comnamp unless it was a builtin. Now the
  | subshell can't call t->comnamp, so it looks for foo and finds
  | the ok foo in the subshell's function tree.

src/cmd/ksh93/bltins/typeset.c:
- Unsetting functions in a virtual/non-forked subshell still
  doesn't work: nv_open() fails to find the function. To work
  around this problem, make 'unset -f' fork the subshell into its
  own process with sh_subfork().
- The workaround exposed another bug: if we unset a function in a
  subshell tree that overrode a function by the same name in the
  main shell, then nv_delete() exposes the function from the main
  shell scope. Since 'unset -f' now always forks a subshell, the
  fix is to simply walk though troot's parent views and delete any
  such zombie functions as well. (Without this, the 4 'more fun'
  tests in tests/subshell.sh fail.)

src/cmd/ksh93/sh/subshell.c: sh_subfuntree():
- Fix function (re)definitions and unsetting in "shared" command
  substitutions of the form '${ commandlist; }' (i.e.: if
  sp->shp->subshare is true). Though internally this is a weird
  form of virtual subshell, the manual page says it does not
  execute in a subshell (meaning, all changes must survive it), so
  a subshell function tree must not be created for these.

src/cmd/ksh93/tests/subshell.sh:
- Add regression tests related to these bugfixes. Test unsetting
  and redefining a function in all three forms of virtual subshell.

(cherry picked from commit dde387825ab1bbd9f2eafc5dc38d5fd0bf9c3652)

src/cmd/ksh93/bltins/typeset.c

@@ -1171,6 +1171,12 @@ static int unall(int argc, char **argv, register Dt_t *troot, Shell_t* shp)
 	while(r = optget(argv,name)) switch(r)
 	{
 		case 'f':
+			/*
+			 * Unsetting functions in a virtual/non-forked subshell doesn't work due to limitations
+			 * in the subshell function tree mechanism. Work around this by forking the subshell.
+			 */
+			if(shp->subshell && !shp->subshare)
+				sh_subfork();
 			troot = sh_subfuntree(1);
 			break;
 		case 'a':
@@ -1253,7 +1259,18 @@ static int unall(int argc, char **argv, register Dt_t *troot, Shell_t* shp)
 			if(troot==shp->var_tree && shp->st.real_fun && (dp=shp->var_tree->walk) && dp==shp->st.real_fun->sdict)
 				nv_delete(np,dp,NV_NOFREE);
 			else if(isfun && !(np->nvalue.rp && np->nvalue.rp->running))
+			{
 				nv_delete(np,troot,0);
+				/*
+				 * If we have just unset a function in a subshell tree that overrode a function by the same
+				 * name in the main shell, then the above nv_delete() call incorrectly restores the function
+				 * from the main shell scope. So walk though troot's parent views and delete any such zombie
+				 * functions. Note that this only works because 'unset -f' now forks if we're in a subshell.
+				 */
+				Dt_t *troottmp;
+				while((troottmp = troot->view) && (np = nv_search(name,troottmp,0)) && is_afunction(np))
+					nv_delete(np,troottmp,0);
+			}
 #if 0
 			/* causes unsetting local variable to expose global */
 			else if(shp->var_tree==troot && shp->var_tree!=shp->var_base && nv_search((char*)np,shp->var_tree,HASH_BUCKET|HASH_NOSCOPE))