all: prepare for new io/fs interfaces

[sbinet]

as mentioned in https://github.com/golang/go/issues/41190, a new file system interface is being designed (and accepted):

• https://go.googlesource.com/proposal/+/master/design/draft-iofs.md

we should probably make use of it or, at least, make sure we implement these interfaces. obvious places:

• xrootd
• perhaps groot