Revert "Merge branch 'xdg_sessions' into devel"

This reverts commit 990fa327a3032ca4b0d6060bffebb8619bb103cc, reversing
changes made to 219100fc8b242bd4f2c9188a37fa79b2d2d9e6bb.

The approach I have chosen, etc. flagging processes that are members of
an user session, was too hacky. Better approach is being implemented in
feature/session branch and there is already a better way to determine
a process session - U_PROC.usession_id - and more facilities are on the
way. Therefore I am discarding the xdg_sessions branch.
fix o-s-d mappings broken by f1a445cd61f44397b7a516fb7648a7703cc4ef4f
Merge branch 'master' into devel
u_module_load_directory(): don't die if module could not be loaded

Just print warning describing the actual error.