Let a group share the info API too, and serve it on windows

The info API is for other programs on the host, and on unix the only
setting between "the user nebula runs as" and "everyone" was a manual
chgrp that the next bind undid; on windows it was not served at all,
because which accounts a pipe admits had not been decided. The control
socket's `group` decides it, and the info API takes the same key.

- `local_api.info.group`: the socket is handed to the group at 0660
  unless `socket_mode` says otherwise, and a mode that gives a named
  group nothing is refused with the mode that would
- windows: served on a named pipe, the group an ACE on it as on the
  control pipe; the e2e test's helpers dial unix sockets, so it still
  skips there
- a changed group or mode on a reload is applied to the socket in place

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

localapi/wire.go

@@ -253,8 +253,9 @@ func wireInfo(ctx context.Context, l *slog.Logger, c *config.C, reg *commands.Re
 
 // errEndpointHeld is returned by listen when the endpoint's name is already held by
 // some other process, which nebula will not serve alongside: whoever holds it would be
-// answering nebula ctl. It is only raised on windows, where the default name can be
-// held by another administrator's process and an override outside the protected
-// namespace by any account's; on unix a path that is still being served is an earlier
-// nebula, which clearStale reports as in use.
-var errEndpointHeld = errors.New("the local control socket's name is held by another process")
+// answering the endpoint's clients. It is only raised on windows, where the default
+// name can be held by another administrator's process and an override outside the
+// protected namespace by any account's; on unix a path that is still being served is an
+// earlier nebula, which clearStale reports as in use. The log line that carries it names
+// the endpoint, so the sentence does not.
+var errEndpointHeld = errors.New("the pipe's name is held by another process")