contrib describe_sessions: Look for sessions in XDG_RUNTIME_DIR
contrib describe_sessions: Look for sessions in XDG_RUNTIME_DIR

The sessions are stored in `$XDG_RUNTIME_DIR` when that variable
is defined.

jkonecny12/kakoune,occivink/kakoune,Somasis/kakoune,lenormf/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,danr/kakoune,mawww/kakoune,alexherbo2/kakoune,jkonecny12/kakoune,Somasis/kakoune,alexherbo2/kakoune,casimir/kakoune,lenormf/kakoune,occivink/kakoune,mawww/kakoune,occivink/kakoune,casimir/kakoune,danr/kakoune,casimir/kakoune,alexherbo2/kakoune,casimir/kakoune,lenormf/kakoune,danr/kakoune,Somasis/kakoune,Somasis/kakoune,occivink/kakoune,lenormf/kakoune,jkonecny12/kakoune,danr/kakoune,mawww/kakoune,mawww/kakoune