bug#21713: On CIFS, mv behaves as mv -f

[Sam Kuper]

On a system where `df -T` shows the file system to be "cifs"
(presumably the Common Internet File System from Microsoft:
https://technet.microsoft.com/en-us/library/cc939973.aspx ), running
`mv` causes unexpected behaviour. Essentially, `mv` behaves as though
`mv -f` had been used.

Example, using GNU Coreutils 8.21 on Ubuntu 14.04.3 LTS:

$ which mv
/bin/mv
$ ls -l
total 0
$ echo foo > 1; chmod -w 1; cp 1 2; ls -l | cut -d' ' -f 1-5,9
-r-x------ 1 me me 4 1
-r-x------ 1 me me 4 2
$ echo bar > 2
-bash: 2: Permission denied
$ mv 1 2
$ ls -l | cut -d' ' -f 1-5,9
-r-x------ 1 me me 4 2

I would have expected the `mv 1 2` command to have prompted the user
before overwriting file 2. It would be helpful to the user if mv could
be improved so that it behaves as expected, even on a "cifs" file
system.

For comparison, running the same commands on a machine with an ext4
file system and a recent version of Coreutils yielded:

$ mv 1 2
mv: replace ‘2’, overriding mode 0444 (r--r--r--)?

as expected.

N.B. I first mentioned this issue at
http://unix.stackexchange.com/q/237123/6860 and am grateful for the
helpful feedback from the people who commented there, which helped me
identify the file system as the likely confounding factor.

Thank you for maintaining Coreutils!

Sam