Update systemd support from several GSD commits

Msg: Backout change from using free() to g_free() in systemd patch
Commit: f1f7bddf007c950608f5f8c5a771a7fb2ab187d5
From: Chris Kühl <chrisk@openismus.com>

Msg: Require systemd 38 or greater when using --enable-systemd
Commit: ff551c6974cfa0801a368ee5c2406ce941ac8cab
From: Chris Kühl <chrisk@openismus.com>

Msg: build: Disallow building with systemd v43 due to C++ incompatibility
Commit: 8a17cd6ff6b0ab4979c6ed8426724ce355d23679
From: Chris Kühl <blixtra@gmail.com>

Msg: Check for logind, not for systemd
Commit: 4cf87daa48a8ab606afe5e3c2ac54c8f6276ba98
From: Martin Pitt <martinpitt@gnome.org>