bug#47253: network-manager shepherd services does not wait to be online

[raid5atemyhomework]

I have a small number of daemons that need access to the network at startup.  I 
have configured their Shepherd services to require `networking`.

However, to my puzzlement, I consistently find that they are unable to access 
the network at startup.  One daemon dies (and gets respawned so often that it 
sometimes gets disabled by Shepherd), the other daemon just keeps running 
without having set up the server that I need it to expose.

Thus, in many cases whenever I reboot I have to manually `herd enable` and 
`herd restart` the first daemon and `herd restart` the second.  This is fairly 
bad since I want to be able to leave this server alone and have it survive 
power interruptions etc.

Checking on other systems, I stumbled on this file on SystemD-based systems:

```systemd
[Unit]
Description=Network Manager Wait Online
Documentation=man:nm-online(1)
Requires=NetworkManager.service
After=NetworkManager.service
Before=network-online.target

[Service]
Type=oneshot
ExecStart=/usr/bin/nm-online -s -q --timeout=30
RemainAfterExit=yes

[Install]
WantedBy=network-online.target
```

Searching the Guix source code, I can't find any `nm-online` command.  So I 
think that, when using the `network-manager` service type, the `networking` 
provision is considered started even though networking isn't actually online 
yet.

I would like to propose this change:

```diff
--- a/gnu/services/networking.scm
+++ b/gnu/services/networking.scm
@@ -1106,17 +1106,22 @@ and @command{wicd-curses} user interfaces."
               (documentation "Run the NetworkManager.")
               (provision '(networking))
               (requirement '(user-processes dbus-system wpa-supplicant 
loopback))
-              (start #~(make-forkexec-constructor
-                        (list (string-append #$network-manager
-                                             "/sbin/NetworkManager")
-                              (string-append "--config=" #$conf)
-                              "--no-daemon")
-                        #:environment-variables
-                        (list (string-append "NM_VPN_PLUGIN_DIR=" #$vpn
-                                             "/lib/NetworkManager/VPN")
-                              ;; Override non-existent default users
-                              "NM_OPENVPN_USER="
-                              "NM_OPENVPN_GROUP=")))
+              (start #~(let ((constructor   (make-forkexec-constructor
+                                              (list (string-append 
#$network-manager
+                                                                   
"/sbin/NetworkManager")
+                                                    (string-append "--config=" 
#$conf)
+                                                    "--no-daemon")
+                                              #:environment-variables
+                                              (list (string-append 
"NM_VPN_PLUGIN_DIR=" #$vpn
+                                                                   
"/lib/NetworkManager/VPN")
+                                                    ;; Override non-existent 
default users
+                                                    "NM_OPENVPN_USER="
+                                                    "NM_OPENVPN_GROUP="))))
+                         (lambda args
+                           (let ((pid (apply constructor args)))
+                             (invoke/quiet (string-append #$network-manager 
"/bin/nm-online")
+                                           "-s" "-q" "--timeout=30")
+                             pid))))
               (stop #~(make-kill-destructor))))))))

 (define network-manager-service-type
```


Of course, the big problem is that Shepherd is single-threadded and `nm-online` 
will block all other bootup.