UnixFS, some missing commands, gofumpt

[mdouchement]

Go format with gofumpt tool.

UnixFS VFS implemented. Change of the signature of some functions from the server package to match Golang's patterns. Move backend to a dedicated package.

New supported functionalities (at least with UnixFS):

• Chown
• Rename
• Link
• Symlink
• Readlink

Tests with NFStest 3.2 with the following command nfstest_posix --server localhost --nfsversion=4 --notty --createlog:

• access: 37/58 pass
• chdir: 6/6 pass
• chmod: 2/16 pass
• close: 4/4 pass
• closedir: 1/1 pass
• creat: 8/8 pass
• fcnlt: 4/4 pass
• fcntl: 30/34 pass
• fdatasync: 2/2 pass
• fstat: 42/42 pass
• fstatvfs: 22/22 pass
• fsync: 2/2 pass
• link: 6/6 pass
• lseek: 10/10 pass
• lstat: 42/42 pass
• mkdir: 8/8 pass
• mmap: 15/15 pass
• munmap: 13/13 pass
• open: 30/31 pass
• opendir: 2/2 pass
• read: 5/5 pass
• readdir: 3/3 pass
• readlink: 9/9 pass
• rename: 14/14 pass
• rewinddir: 4/4 pass
• rmdir: 6/6 pass
• seekdir: 5/5 pass
• stat: 42/42 pass
• statvfs: 22/22 pass
• symlink: 4/4 pass
• sync: 2/2 pass
• telldir: 5/5 pass
• unlink: 12/12 pass
• write: 10/10 pass

[smallfz]

Thank you for this pr! UnixFS is great as it is what I need in next days.

[smallfz]

I use gofmt a lot but I see gofumpt generates more clear and structured results. I'll switch to gofumpt for this project.