Fix 'unset -f' to work in subshells without forking (re: 047cb330)

This commit implements unsetting functions in virtual subshells,
removing the need for the forking workaround. This is done by
either invalidating the function found in the current subshell
function tree by unsetting its NV_FUNCTION attribute bits (which
will cause sh_exec() to skip it) or, if the function exists in a
parent shell, by creating an empty dummy subshell node in the
current function tree without that attribute.

As a beneficial side effect, it seems that bug 228 (unset -f fails
in forked subshells if a function is defined before forking) is now
also fixed.

src/cmd/ksh93/include/defs.h,
src/cmd/ksh93/sh/init.c:
- Add sh.fun_base for a saved pointer to the main shell's function
  tree for checking when in a subshell, analogous to sh.var_base.

src/cmd/ksh93/bltins/typeset.c: unall():
- Remove the fork workaround.
- When unsetting a function found in the current function tree
  (troot) and that tree is not sh.var_base (which checks if we're
  in a virtual subshell in a way that handles shared-state command
  substitutions correctly), then do not delete the function but
  invalidate it by unsetting its NV_FUNCTION attribute bits.
- When unsetting a function not found in the current function tree,
  search for it in sh.fun_base and if found, add an empty dummy
  node to mask the parent shell environment's function. The dummy
  node will not have NV_FUNCTION set, so sh_exec() will skip it.

src/cmd/ksh93/sh/subshell.c:
- sh_subfuntree(): For 'unset -f' to work correctly with
  shared-state command substitutions (subshares), this function
  needs a fix similar to the one applied to sh_assignok() for
  variables in commit 911d6b06. Walk up on the subshells tree until
  we find a non-subshare.
- sh_subtracktree(): Apply the same fix for the hash table.
- Remove table_unset() and incorporate an updated version of its
  code in sh_subshell(). As of ec888867, this function was only
  used to clean up the subshell function table as the alias table
  no longer exists.
- sh_subshell():
  * Simplify the loop to free the subshell hash table.
  * Add table_unset() code, slightly refactored for readability.
    Treat dummy nodes now created by unall() separately to avoid a
    memory leak; they must be nv_delete()d without passing the
    NV_FUNCTION bits. For non-dummy nodes, turn on the NV_FUNCTION
    attribute in case they were invalidated by unall(); this is
    needed for _nv_unset() to free the function definition.

src/cmd/ksh93/tests/subshell.sh:
- Update the test for multiple levels of subshell functions to test
  a subshare as well. While we're add it, add a very similar test
  for multiple levels of subshell variables that was missing.
- Add @JohnoKing's reproducer from #228.

src/cmd/ksh93/tests/leaks.sh:
- Add leak tests for unsetting functions in a virtual subshell.
  Test both the simple unset case (unall() creates a dummy node)
  and the define/unset case (unall() invalidates existing node).

Resolves: https://github.com/ksh93/ksh/issues/228

src/cmd/ksh93/include/defs.h

@@ -165,6 +165,7 @@ struct shared
 	int		path_err;	/* last error on path search */ \
 	Dt_t		*track_tree;	/* for tracked aliases */ \
 	Dt_t		*var_base;	/* global level variables */ \
+	Dt_t		*fun_base;	/* global level functions */ \
 	Dt_t		*openmatch; \
 	Namval_t	*namespace;	/* current active namespace */ \
 	Namval_t	*last_table;	/* last table used in last nv_open */ \