If the init process (pid 1) is systemd, set class 'systemd'

This commit checks /proc/1/cmdline on Linux and tries to determine if
its running systemd. Handle SUSE (symlink) and RHEL (arguments) cases
correctly.

On SUSE, /proc/1/cmdline reads
  "/sbin/init" which is a symlink to /usr/lib/systemd/systemd.

On RHEL, /proc/1/cmdline reads
  "/usr/lib/systemd/systemd --switched-root --system--deserialize23"
inventory/linux.cf: minor fixes