Directory::get_space_left() returns invalid result on Unix

[akien-mga]

Godot version:
3.x (c0b3ad2)
master (3b380f4)

OS/device including version:
Linux, Mageia Cauldron x86_64
According to @bruvzg, also reproducible on macOS

Issue description:
Running Directory::get_space_left() seems to return garbage data, at least on x86_64.
The data does not correspond to the available disk space reported by df.

For example, with this script:

var dir = Directory.new()
dir.open("/")
print(dir.get_space_left())
dir.open("/home")
print(dir.get_space_left())

And this partition setup:

$ df
[akien@cauldron Over9000]$ df
Filesystem      Size  Used Avail Use% Mounted on
...
/dev/nvme0n1p6   59G   46G   11G  82% /
...
/dev/nvme0n1p7  196G  173G   22G  89% /home

I get, on master and 3.x with x86_64 build:

1208348672
-481312768

Earlier today I had:

1208475648
222502912

I've been compiling the Rust compiler in the meantime which can explain the diff between the values for /home.

With an i686 build, it seems better:

14093234176
25292083200

Steps to reproduce:

• Compare the output of Directory.get_space_left() on your system to what the system reports

Minimal reproduction project:
OS/user-dependent, but see above for a snippet.

[bruvzg]

On macOS using vfs.f_frsize (fundamental file system block size) instead of vfs.f_bsize (file system block size) works.

Also, it might be better to use vfs.f_bavail (space available to user) instead of vfs.f_bfree (space available to system).
In case of macOS it will be smaller for the system volume, excluding APFS snapshots.

[bruvzg]

(uint64_t)vfs.f_bavail * (uint64_t)vfs.f_frsize also works on both 32-bit and 64-bit Linux.