librda fails to detect local session managed by systemd

[peat-psuwit]

Version: 0.0.5-1 in Ubuntu 20.04, 0.0.5-1.1 in Ubuntu 23.04

librda is hard-coded to look for $XDG_SEAT environment variable for local session signal. However, nowadays almost all user session components are managed using Systemd, and by convention (?), this environment variable is not imported into Systemd environment variables [1].

This means, for Systemd-managed local session, librda will not say the session is local, and ironically will not way it's remote either. This is tested on Ubuntu Touch 20.04 and Ubuntu MATE 23.04.

This causes problem when testing ayatana-indicator-power 23.6.1 into Ubuntu Touch, as the indicator header becomes invisible if librda says the session is not local.

[1] Debian did this in XSession's 95dbus_update-activation-env, which I copied for Ubuntu Touch in ubuntu-touch-session session leader script. I have to admit I have no idea why.