session: Only append session manager if supported by gnome-session
gnome-session dropped support for --builtin and --systemd. So don't append that argument if it doesn't show up in gnome-session's help output.
This is what we have in 0.37.1 and we don't want main
to lag behind.
Signed-off-by: Guido Günther agx@sigxcpu.org Part-of: !1387 (merged)