This allows us to skip storing the actual backing data in a separate
`Vector`, as we know that we are working with `environ`-backed storage
here.
Also, while the logic is currently very similar to what `getenv` does
internally, this allows us to eventually implement custom environment
variable filters while remaining linear in run time.
This allows using pls on a program with arguments more ergonomically,
e.g. `pls -- echo "hello friends"` can now simply be done as:
`pls echo "hello friends"`.
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.
