dbus: add NewSystemdConnection() for connecting directly to systemd.
dbus: add NewSystemdConnection() for connecting directly to systemd.

Systemd exposes a dbus-like interface at /run/systemd/private which is
just dbus without the auth/hello.

Using this connection Go programs may be written which utilize systemd's
dbus integration without requiring a dbus daemon.

glerchundi/go-systemd,jonboulle/go-systemd,Yhgenomics/go-systemd,faiq/go-systemd,faiq/go-systemd,dnaeon/go-systemd,mikkeloscar/go-systemd,jareksm/go-systemd,brnv/go-systemd,coreos/go-systemd,pquerna/go-systemd,endocode/go-systemd,jareksm/go-systemd,ZeissS/go-systemd,endocode/go-systemd,pcarranzav/go-systemd,pcarranzav/go-systemd,brnv/go-systemd,pquerna/go-systemd,jonboulle/go-systemd,glerchundi/go-systemd,mikkeloscar/go-systemd,ZeissS/go-systemd,Yhgenomics/go-systemd,coreos/go-systemd,dnaeon/go-systemd