main: signal readiness by notifying systemd

This is useful if we want to be able to start some services depending
on i3, notably some script using the socket or third-party bars like
polybar. To make use of this change, user is expected to use the
following stanza:

```
[Unit]
Description=i3 window manager
PartOf=graphical-session.target

[Service]
Type=notify
ExecStart=/usr/bin/i3
ExecStopPost=/bin/systemctl --user stop graphical-session.target
Restart=on-failure
```

Something similar is already possible using socket activation. For
example, we could use:

```
[Unit]
Description=i3 window manager socket
PartOf=graphical-session.target

[Socket]
ListenStream=%t/i3.sock
ExecStartPost=/bin/systemctl --user set-environment I3SOCK=%t/i3.sock
```

And other units could `Requires=i3.socket`. Unfortunately, not
everything is using I3SOCK. Notably, `i3 --get-socketpath` does not
and that's what i3ipcpp is doing. An alternative would be to patch `i3
--get-socketpath` to use I3SOCK if present, however, this may be a bit
risky. Should we check the environment variable first or the root
attribute?

Another alternative not requiring any modification is to have a
dedicated `i3-session.target`:

```
[Unit]
Description=i3 session
BindsTo=graphical-session.target
Wants=wallpaper.service
Wants=wallpaper.timer
Wants=polybar.service
Wants=i3-companion.service
```

And trigger it from i3:

```
exec_always --no-startup-id systemctl --user start --no-block i3-session.target
```

The proposed change being quite small, it seems harmless and
low-maintenance.

Signed-off-by: Vincent Bernat <vincent@bernat.ch>

RELEASE-NOTES-next

@@ -45,6 +45,7 @@ option is enabled and only then sets a screenshot as background.
   • Add 'all' window matching criterion
   • Acquire the WM_Sn selection when starting as required by ICCCM
   • Add --replace command line argument to replace an existing WM
+  • Notify systemd when i3 is ready, allowing other services to use i3 as a dependency
 
  ┌────────────────────────────┐
  │ Bugfixes                   │

src/main.c

@@ -1010,9 +1010,9 @@ int main(int argc, char *argv[]) {
         ev_io_start(main_loop, log_io);
     }
 
-    /* Also handle the UNIX domain sockets passed via socket activation. The
+    /* Also handle the UNIX domain sockets passed via socket
-     * parameter 1 means "remove the environment variables", we don’t want to
+     * activation. The parameter 0 means "do not remove the
-     * pass these to child processes. */
+     * environment variables", we need to be able to reexec. */
     listen_fds = sd_listen_fds(0);
     if (listen_fds < 0)
         ELOG("socket activation: Error in sd_listen_fds\n");
@@ -1196,5 +1196,6 @@ int main(int argc, char *argv[]) {
      * when calling exit() */
     atexit(i3_exit);
 
+    sd_notify(1, "READY=1");
     ev_loop(main_loop, 0);
 }