Add support for >2GB downloads

[mathiascode]

Support for large files was added to SoulseekQt in 2015, but parsing a TransferRequest response in Nicotine+ only returns 0 as the filesize for large files. Investigate what the official client does different.

Edit: Should also check if Museek+ supports large downloads.

[mathiascode]

It is worth noting that large uploads seem to work just fine (tested with 16GB file).

[mathiascode]

The only work left to do is to is to figure out why large downloads from SoulseekQt clients return a file size of 0. >2GB file transfers between Nicotine+ clients and uploads to SoulseekQt clients will work fine when Nicotine+ 2.0.0 is released.

[mathiascode]

Fully supported in #328, we just ignore the invalid file size sent by SoulseekQt.

Summary of changes:
b4304fa
1239785

[kiplingw]

My guess is it probably involves a primitive whose width is too small to represent the value.

[mathiascode]

What's also curious is that Nicotine+ correctly shows the size of >2GB files in search results, while SoulseekQt shows no size. SoulseekQt shows the correct size if you browse a user share though. :P

[mathiascode]

Heh, now I realized that someone already came up with the same workaround in Museekd. eLvErDe/museek-plus@fd12d21