Further UserDefinedFileAttributeView improvements and fixes

[Alan Bateman]

On 10/03/2021 16:42, Sebastian Stenzel wrote:
> I started working on JDK-8262958 as well, now. Since it will contain multiple changes that build upon each other, I want to ask how to deal with it.
>
> My suggestion: I keep this PR a draft for now and have you review each change for itself. Or does it _need_ to be marked RFR to spit out individual webrevs?
Small changes are usually easier to review. So if it can be broken up in 
a sequence of small changes, each with its own JBS and PR, then it would 
be easier I think.

-Alan