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
