stage1/init: check for systemd before invoking RunningFromSystemService
stage1/init: check for systemd before invoking RunningFromSystemService

Currently if RunningFromSystemService is invoked on a non-systemd system
this call fails with "error calling sd_pid_get_owner_uid".

This fixes it by checking if we are running systemd at all.

intelsdi-x/rkt,coreos/rkt,Shashwatsh/rkt,intelsdi-x/rkt,achanda/rkt,jakon89/rkt,rkt/rkt,shobhit85/rkt,jonboulle/rkt,rkt/rkt,cab105/rkt,joshix/rkt,rkt/rkt,dgonyeo/rkt,dgonyeo/rkt,ybubnov/rkt,alban/rkt,jakon89/rkt,joshix/rkt,sstabellini/rkt,coreos/rkt,achanda/rkt,ybubnov/rkt,jonboulle/rkt,sstabellini/rkt,lucasallan/rkt,Shashwatsh/rkt,kinvolk/rkt,lucasallan/rkt,shobhit85/rkt,alban/rkt,lucasallan/rkt,kinvolk/rkt,joshix/rkt,jonboulle/rkt,ybubnov/rkt,cab105/rkt,coreos/rkt,jakon89/rkt,dgonyeo/rkt,intelsdi-x/rkt,achanda/rkt,joshix/rkt,alban/rkt,Shashwatsh/rkt,kinvolk/rkt,lucasallan/rkt,sstabellini/rkt,kinvolk/rkt,cab105/rkt,shobhit85/rkt,rkt/rkt