(posix_)fallocate where possible and sane

[RealDolos]

Telling the file system how big a file will be enables it to better place files on disk, avoid fragmentation, etc. *nix have a mechanism to effectively reserve a file of a certain size in the posix_fallocate or linux specific fallocate APIs. New file system will allocate a file almost instantly. However, "old" file systems like ext3 will actually write zeros, but who still uses ext3, r-right?! Windows has no equivalent API...

[Megous]

Sadly, this would throw off the resume logic, which depends on the size of a file to determine how much data was already downloaded and where to resume after user cancels and restarts the process.

[RealDolos]

Oh right, I remember disabling resume for that reason... still would be nice to have, tho that would mean tracking resume information explicitly some place else.