feat(ldap): Start and configure only if enabled.

prosody/rootfs/etc/services.d/10-saslauthd/run

@@ -1,2 +1,8 @@
 #!/usr/bin/with-contenv bash
-exec s6-setuidgid root saslauthd -a ldap -O /etc/saslauthd.conf -c -m /var/run/saslauthd -n 5 -d
+if [[ -f /etc/saslauthd.conf ]]; then
+  exec s6-setuidgid root saslauthd -a ldap -O /etc/saslauthd.conf -c -m /var/run/saslauthd -n 5 -d
+else
+    # if saslauthd should not be started,
+    # prevent s6 from restarting this script again and again
+    s6-svc -O /var/run/s6/services/10-saslauthd
+fi