pstree: Single-tree display of processes filtered by user

Current pstree can do two things, but separately: filter processes by user, and display a tree of processes even if not all of them can be actually shown.

The former displays disconnected subtrees, each having their own root. The latter occurs in case the user can see only their own processes in /proc, everything is displayed as a single tree, and nodes that don't belong to the user are replaced by a ?.

Now, I'd find it very helpful to get processes filtered by user shown as such a single-tree display, with nodes that are needed to connect all leaves to the root replaced by that ?. I'd personally prefer this to be the default but if people rely on further processing the disconnected output, single-tree display should at least be an option.