Fix for #30 - Use sleep inifinity instead of sleep <number>

Apparently using sleep with very large numbers is not a good idea
when your image can be compiled down to 32-bit platforms.

This fix uses `sleep infinity` and -- failing that -- tries to
sleep for about 68 years (which is exactly 2^15 seconds).

scripts/opendkim.sh

@@ -1,8 +1,17 @@
 #!/bin/sh
+
+noop() {
+    while true; do
+        # 2147483647 = max signed 32-bit integer
+        # 2147483647 s ≅ 70 years
+        sleep infinity || sleep 2147483647
+    done
+}
+
 if [ ! -d /etc/opendkim/keys ]; then
-    sleep 9999999999999999999
+    noop
 elif [ -z "$(find /etc/opendkim/keys -type f ! -name .)" ]; then
-    sleep 9999999999999999999
+    noop
 else
     /usr/sbin/opendkim -D -f -x /etc/opendkim/opendkim.conf
 fi