Bug 2910

What I'm seeing:
----------------
When a user login hits a "Match" section, the pam modules seem to be unable to use stdout.

Steps to repro:
---------------
I'm running an almost vanilla openssh installation inside a ubuntu:bionic container. In here, I append the following to the bottom of the file:

```
Match Group foobar
	ForceCommand bash

```
Next, I add 2 users to the system, named `foobar` and `bazbaz` .

Next, I add some text to the MOTD file (/etc/motd).

Next, I also modify the /etc/pam.d/sshd config to contain these:
```
session    optional     pam_motd.so noupdate
session    optional     pam_exec.so stdout /bin/echo ECHOECHO
```

So when I log in using `bazbaz` I see that the motd and the stdout message appears (as emitted by the PAM module). Also the PrintLastLog directive kicks in and the last login is also printed.

However when I log in using `foobar` (where the Match directive kicks in) then I see neither the motd, nor the stdout message, nor the lastlog. This leads me to believe that stdout handling is somehow different when the Match is hit. I would have suspected the pam modules (or libpam) but since the LastLog is an `sshd` function, I'm inclined to believing this is somehow related to stdout (hence marking the component as 'sshd').

Not sure if this is a bug or a case of missing documentation.