Added option to specify the minimum disk space at which recording is stopped

We record on a small-ish disk (10GB available) and we've been bitten several time by the hard-coded 1GB limit at which recording stops. On our system, is is 10% of the capacity that is lost. This option makes this parameter manually configurable for a given system.
I thought that was the point of the default value in the option description, but over-initializing does not hurt.