Detect util-linux version of runuser

Alternative is to always use runuser invocation in else clause.

Fixes #44.
[#150221349]

scripts/rabbitmq-script-wrapper

@@ -77,7 +77,18 @@ exec_script_as_rabbitmq() {
 exec_script_as_root() {
   if [ -x /sbin/runuser ]
   then
-    exec /sbin/runuser -u rabbitmq -- "/usr/lib/rabbitmq/bin/$SCRIPT" "$@"
+    # TODO:
+    # At some point all of the RabbitMQ supported distributions will be using
+    # the util-linux version of /sbin/runuser, as it has been removed from GNU
+    # coreutils as of 2012. At that point the first clause of the following
+    # if statement can become the only statement used and the if/then
+    # removed
+    if /sbin/runuser --version | grep -qF util-linux
+    then
+        exec /sbin/runuser -u rabbitmq -- "/usr/lib/rabbitmq/bin/$SCRIPT" "$@"
+    else
+        exec /sbin/runuser -s /bin/sh -- rabbitmq "/usr/lib/rabbitmq/bin/$SCRIPT" "$@"
+    fi
   elif [ -x /bin/su ]
   then
     exec /bin/su -s /bin/sh rabbitmq -- "/usr/lib/rabbitmq/bin/$SCRIPT" "$@"