Improve wayland wm detection

[jcorbin]

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Description

On my arch system running the river wm, get_wm wasn't working.

First off, I'd had no reason to install lsof as yet, having only fuser.

Next up the output of fuser was way more than the expected "just a pid": it included a bunch of permission denied entries while enumerating all non-user procs, and also some extra whitespace around the output pid, which was interacting poorly with the defensive quoting inside the sub-shell expansion of $tmp_pid.

TODO

• [ ] maybe add river to the guess branch when using neither lsof nor fuser?