Fix fork after redirecting stdout in subshare (re: 500757d7)

Previously, command substitutions executed as virtual subshells
were always forked if any command was run within them that
redireceted standard output, even if the redirection was local to
that command.

Commit 500757d7 removed the check for a shared-state command
substitution (subshare), so introduced a bug where even that would
fork, causing it to stop sharing its state.

We can further improve on that fix by only forking if the
redirection is permanent as with `exec` or `redirect`. There should
be no need to do that if the redirection is local to a command run
within the command substitution, as the file descriptor is restored
when that command finishes, which is still within the command
substitution.

src/cmd/ksh93/sh/io.c: sh_redirect():
- Only fork upon redirecting stdout if the virtual subshell is a
  command substitution, and if the redirection is permanent
  (flag==1 or flag==2).

NEWS

@@ -3,6 +3,12 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-04-26:
+
+- Fixed a bug introduced on 2021-02-20 in which a shared-state command
+  substitution stopped sharing its state with the calling shell environment
+  if it executed a command that locally redirected standard outpuut.
+
 2021-04-22:
 
 - shcomp (the shell bytecode compiler) was fixed to correctly compile process