Set 'df' to output in POSIX mode

GNU `df` will add some linebreaks that cause this fail counter to fail.  It will happen if the `filesystem` field in `df` output is too long, and it manifests as emails from `vzfailcnt` about `diskspace` and `diskinodes` with absurd values and subject lines like:

> OpenVZ failcnt increased at hostname.example.com: diskspace.%.53944320

Using `df -P` to force df to output in POSIX standard format fixes this.