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