util: add sessionleader heuristic to service check
util: add sessionleader heuristic to service check

This adds a heuristic to RunningFromSystemService that checks if the
invoking process is a session leader, which should help eliminate false
positives on systems that do not have `pam_systemd` configured.

This is a port of vcaputo's patch to rkt which landed in
https://github.com/coreos/rkt/pull/1694/commits

glerchundi/go-systemd,coreos/go-systemd,glerchundi/go-systemd,coreos/go-systemd,endocode/go-systemd,jareksm/go-systemd,jonboulle/go-systemd,brnv/go-systemd,endocode/go-systemd,faiq/go-systemd,jonboulle/go-systemd,jareksm/go-systemd,faiq/go-systemd,brnv/go-systemd