Remove maxsend limits

This patch allows increasing maxsend to any number as you like, or remove the limit entirely by setting it to -1. This is invaluable for hosters, since limits to download sizes tend to be problematic when picking addons and can even render some addons not viable when doing serious hosting. By removing this limit, hosters won't need to worry about this, and can instead focus on curating a good set of addons for a server.

Since 2.2.14, this is not as relevant because of HTTP download, but since setting up an HTTP server for downloading is a tricky process, removing this limit will still benefit less tech-savvy hosts.

Downstream patch: https://github.com/P-AS/srb2-legacy/pull/71