Feature request: configure to stop downloading when space is low

[livnev]

This is a feature request that applies to any file transfer program, posting it since this is by far the most common cause of running out of disk space for me.

It would be useful to have something like set xfer:minimum-remaining-space 1G which would either cause downloads to be paused when the space remaining on the target is < 1 gigabyte, or refuse to start a download when space remaining after download is calculated to be < 1 gigabyte (or both).

A lot of systems start behaving poorly when the storage is full and I'm not aware of a generic way to prevent that from happening.