Automatically set XDG_SESSION_TYPE based on session file location

[hexchain]

Currently, ReGreet does not set XDG_SESSION_TYPE by default, and it is not possible to set different variables for different sessions.

This can be a problem for some desktop environment components. For example, pam_kwallet5.so checks for a desktop session by checking if XDG_SESSION_TYPE is x11 or wayland, and does nothing if the variable is absent. When this happens the KWallet keyring will not be automatically unlocked on login.

It would be nice if ReGreet could treat xsessions and wayland-sessions differently and set this variable before starting the session (like GDM and SDDM).