pls: Drastically simplify this program

Since this program is setuid-root, it should be as simple as possible.

To that end, remove `/etc/plsusers` and use filesystem permissions to
achieve the same thing. `/bin/pls` is now only executable by `root` or
members of the `wheel` group.

Also remove all the logic that went to great lengths to `unveil()` a
minimal set of filesystem paths that may be used for the command.
The complexity-to-benefit ratio did not seem justified, and I think
we're better off keeping this simple.

Finally, remove pledge promises the moment they are no longer needed.

Base/etc/plsusers

@@ -1,4 +0,0 @@
-# plsusers file
-# Put any users you want to allow to run programs as root here
-root
-anon