Fix Directory::get_space_left() result on macOS and Linux.

[bruvzg]

• Use fragme_size instead of block_size as base unit.
• Use "free space for unprivileged users" instead of "total free space" (which corresponds to the Windows implementation and value shown by df and mc and some other Linux file managers).

Tested on macOS 11.4, Ubuntu 21.04 (x86-64), Mageia 8 (x86-64 and i586) with different file systems.

Fixes #47262

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.4.

[akien-mga]

Cherry-picked for 3.3.3.

[bruvzg]

By the way, is this "Truncate to closest MiB" part of the GDScript binding necessary for something, or it's just an artifact from the broken conversion to MiB? Previously it has comment "return value in megabytes, given binding is int", but code was the same (missing bracket?).

godot/core/core_bind.cpp

Line 1725 in 652a9de

return d->get_space_left() / 1024 * 1024; // Truncate to closest MiB.

[akien-mga]

I changed the comment because the previous one didn't make sense given the code and was too orthogonal to the changes in #47254.

But yeah the intent was maybe actually to return MiB instead of bytes since those are typically big values which could easily be more than INT32_MAX, which I guess was a limit in Godot in the past?

I think we could either remove the truncation and keep bytes (and document it, current classref doesn't say the unit), or actually convert to MiB. The latter is maybe the most user-friendly?