leveldb: prepend user dir to relative db path?

When the leveldb node is given a relative path, e.g. my-dir/my-db, perhaps it should prepend RED.settings.userDir to this path. This is the behavior that I'd expect, anyhow.

Otherwise, leveldown appears to want to use the filesystem root, and will throw an exception if NR is not running as superuser. If NR is running privileged, then the behavior is likely undesirable anyhow.

This should not affect absolute paths such as /home/me/.node-red/my-db. That's a little tedious to type, as well. 😉

I can send a PR if this would be desirable--but it is a breaking change.

(Alternatively, maybe just add some docs to the info pane.)

Start by locating the leveldb node's path handling and how it uses RED.settings.userDir and leveldown. Compare the current behavior for relative and absolute paths, then determine whether the expected result is a path change or documentation in the info pane; done requires an agreed approach and coverage of the selected behavior.