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/tests/subshell.sh

@@ -623,4 +623,34 @@ do	if	[[ -e $f ]]
 	fi
 done
 
+# ======
+# Unsetting or redefining functions within subshells
+
+# ...function can be unset in subshell
+
+func() { echo mainfunction; }
+(unset -f func; typeset -f func >/dev/null 2>&1) && err_exit 'function fails to be unset in subshell'
+v=$(unset -f func; typeset -f func >/dev/null 2>&1) && err_exit 'function fails to be unset in comsub'
+v=${ unset -f func 2>&1; } && ! typeset -f func >/dev/null 2>&1 || err_exit 'function unset fails to survive ${ ...; }'
+
+# ...function can be redefined in subshell
+func() { echo mainfunction; }
+(func() { echo sub; }; [[ ${ func; } == sub ]]) || err_exit 'function fails to be redefined in subshell'
+v=$(func() { echo sub; }; func) && [[ $v == sub ]] || err_exit 'function fails to be redefined in comsub'
+v=${ { func() { echo sub; }; } 2>&1; } && [[ $(PATH=/dev/null func) == sub ]] \
+	|| err_exit 'function redefine fails to survive ${ ...; }'
+
+# ...some more fun from Stéphane Chazelas: https://github.com/att/ast/issues/73#issuecomment-384178095
+a=$tmp/morefun.sh
+cat >| "$a" <<EOF
+true() { echo WRONG; }
+(true() { echo ok; } && true && unset -f true && true) || false
+# the extra '|| false' avoids optimising out the subshell
+EOF
+v=$("$SHELL" "$a") && [[ $v == ok ]] || err_exit 'fail: more fun 1'
+v=$("$SHELL" -c "$(cat "$a")") && [[ $v == ok ]] || err_exit 'fail: more fun 2'
+v=$("$SHELL" -c 'eval "$(cat "$1")"' x "$a") && [[ $v == ok ]] || err_exit "fail: more fun 3"
+v=$("$SHELL" -c '. "$1"' x "$a") && [[ $v == ok ]] || err_exit "fail: more fun 4"
+
+# ======
 exit $((Errors<125?Errors:125))