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/sh/parse.c

@@ -1465,10 +1465,10 @@ static Shnode_t *simple(Lex_t *lexp,int flag, struct ionod *io)
 			{
 				/* check for builtin command */
 				Namval_t *np=nv_bfsearch(argp->argval,lexp->sh->fun_tree, (Namval_t**)&t->comnamq,(char**)0);
-				if(cmdarg==0)
-					t->comnamp = (void*)np;
 				if(np && is_abuiltin(np))
 				{
+					if(cmdarg==0)
+						t->comnamp = (void*)np;
 					if(nv_isattr(np,BLT_DCL))
 					{
 						assignment = 1+(*argp->argval=='a');

src/cmd/ksh93/sh/subshell.c

@@ -402,7 +402,7 @@ Dt_t *sh_subfuntree(int create)
 	register struct subshell *sp = subshell_data;
 	if(!sp || sp->shp->curenv==0)
 		return(sh.fun_tree);
-	if(!sp->sfun && create)
+	if(!sp->sfun && create && !sp->shp->subshare)
 	{
 		sp->sfun = dtopen(&_Nvdisc,Dtoset);
 		dtview(sp->sfun,sp->shp->fun_tree);