getPathVar fails for large files

Joeys reports at http://bugs.debian.org/773821:

```
import System.Posix.Files
import Control.Exception
import Foreign.C.Types
main = print =<< (try (getPathVar "." FileNameLimit) :: IO (Either IOException CLong))
```

This program can fail on a system with a large disk:

```
Left .: getPathVar: failed (Value too large for defined data type)

statfs(".", 0xffa8ad50) = -1 EOVERFLOW (Value too large for defined data type)
```

The problem is, getPathVar is using statfs, not statfs64.

Here's a C program that uses statfs64:

```
#define _LARGEFILE64_SOURCE 1
#include
#include

main () {
struct statfs64 buf;
statfs64(".", &buf);
printf("%li\n", buf.f_namelen);
}
```

And strace shows just how large some of the fields are, which is why
statfs() overflows.

```
statfs64(".", 84, {f_type=0x2fc12fc1, f_bsize=131072, f_blocks=67107265, f_bfree=66904371, f_bavail=66904371, f_files=17127878964, f_ffree=17127519173, f_fsid={-676822403, 15770009}, f_namelen=255, f_frsize=131072, f_flags=4128}) = 0
```

An added problem is that getPathVar is implemented using pathconf,
rather than using statfs directly. It seems that setting _LARGEFILE64_SOURCE
does not cause pathconf to use statfs64. This may be a glibc bug.