Prevent podman varlink socket fight

When enabled, it's desired for the podman-varlink process to startup on
boot or upon socket-activation, whichever happens first.  However,
with `KillMode=none` systemd will never kill any podman-varlink
processes.  This makes it easily possible for multiple podman-varlink
processes to be running, and fight each other to service a single socket.

---
For example:

Prior to this commit, this will result in four podman-varlink processes
being run:

```
systemctl enable io.podman.socket
systemctl enable io.podman.service
systemctl start io.podman.socket
systemctl start io.podman.service
systemctl start io.podman.service
```

Fix this by setting `KillMode=process` and `TimeoutStopSec=30` (default
is 90).  This results in podman-varlink exiting on its own after a minute
of being idle (--timeout=60000).  Alternatively, systemd will manage the
service stop by sending a SIGTERM, then if podman-varlink has not exited
within `TimeoutStopSec`, a SIGKILL will be sent.

Signed-off-by: Chris Evich <cevich@redhat.com>

contrib/varlink/io.podman.service

@@ -6,8 +6,9 @@ Documentation=man:podman-varlink(1)
 
 [Service]
 Type=simple
-ExecStart=/usr/bin/podman varlink unix:%t/podman/io.podman
-KillMode=none
+ExecStart=/usr/bin/podman varlink unix:%t/podman/io.podman --timeout=60000
+TimeoutStopSec=30
+KillMode=process
 
 [Install]
 WantedBy=multi-user.target

contrib/varlink/io.podman.socket

@@ -8,3 +8,4 @@ SocketMode=0600
 
 [Install]
 WantedBy=sockets.target
+Also=multi-user.target