systemd: Do not kill user sessions by default

From the manual page logind.conf(5):

> KillUserProcesses=
>     Takes a boolean argument. Configures whether the processes of a
>     user should be killed when the user logs out. If true, the scope
>     unit corresponding to the session and all processes inside that
>     scope will be terminated. If false, the scope is "abandoned", see
>     systemd.scope(5), and processes are not killed. Defaults to "yes",
>     but see the options KillOnlyUsers= and KillExcludeUsers= below.
>
>     In addition to session processes, user process may run under the
>     user manager unit user@.service. Depending on the linger settings,
>     this may allow users to run processes independent of their login
>     sessions. See the description of enable-linger in loginctl(1).
>
>     Note that setting KillUserProcesses=yes will break tools like
>     screen(1) and tmux(1), unless they are moved out of the session
>     scope. See example in systemd-run(1).

systemd.be0

@@ -67,6 +67,7 @@ mee_configure() {
         -Drootlibdir=/lib \
         -Dsplit-usr=true \
         -Dsysusers=false \
+        -Ddefault-kill-user-processes=false \
         -Db_lto=false
 }