DirFileSystem.mv should call the underlying fs

[nilshamerlinck]

• The issue was discussed here: Error on mv using DirFileSystem with fs=SMBFileSystem #1335
• Also taking the opportunity to rename mv_file into mv where needed (as mv_file is not defined in the interface of AbstractFileSystem, while mv is) but keeping mv_file as a copy of mv for retro-compatibility in case someone is using it

[martindurant]

Does anything call mv_file? In most cases, the _file means it operates on a single file at a time, whereas the one without does possible lists and glob expansion. Perhaps we need both?

[martindurant]

Since mv_file is not in the spec, and by convention should not mean the same as mv, I would consider removing it wherever it appears.

[nilshamerlinck]

ok @martindurant, done in #1585