processpipe: allow synchronous close to set $?
To get rid of the ugly $PublicInbox::DS::in_loop localization
in MboxReader, we'll distinguish between ->CLOSE and ->DESTROY
with ProcessPipe.
If we end up closing via ->DESTROY, we'll assume the caller will
want to deal with $? asynchronously via the event loop (or not
even care about $?).
If we hit ->CLOSE directly, we'll assume the caller called
close() and wants to check $? synchronously.
Note: wantarray doesn't seem to propagate into tied methods,
otherwise I'd be relying on that.