Provide a way of determining init system used

dib-init-system script is installed into $PATH. Called
without arguments it will print the name of init system
used to stdout.

Additionally, set DIB_INIT_SYSTEM environment variable to
the init system used.

Tested on ubuntu+upstart, centos+upstart, fedora+systemd,
debian+sysv.

Closes-Bug: #1251610

Change-Id: I29668079091f6060dab66d8259890384d3bbd653

elements/base/dib-init-system

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+set -eu
+
+if [ -f /sbin/initctl ]; then
+    echo "upstart"
+elif [ -f /usr/bin/systemctl ]; then
+    echo "systemd"
+elif [ -f /sbin/init ]; then
+    if [ -f /bin/systemd ]; then
+        echo "systemd"
+    else
+        echo "sysv"
+    fi
+else
+    echo "Unknown init system"
+    exit 1
+fi

elements/base/environment.d/10-dib-init-system.bash

@@ -0,0 +1 @@
+export DIB_INIT_SYSTEM=$(dib-init-system)

elements/base/pre-install.d/04-dib-init-system

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+set -e
+
+install -m 0755 -o root -g root $(dirname $0)/../dib-init-system /usr/bin/