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)

TODO

@@ -54,11 +54,6 @@ https://github.com/modernish/modernish/tree/0.16/lib/modernish/cap/
   not work within the command substitution, acting as if standard output is
   still closed.
 
-- BUG_FNSUBSH: Function definitions within subshells (including command
-  substitutions) are ignored if a function by the same name exists in the
-  main shell, so the wrong function is executed. 'unset -f' is also silently
-  ignored. This only applies to non-forked subshells.
-
 - BUG_IFSGLOBS: In glob pattern matching (as in case or parameter
   substitution with # and %), if IFS starts with ? or * and the "$*"
   parameter expansion inserts any IFS separator characters, those characters