Add a tool for formatting floppy disks or rewritable optical media

[uweseimet]

With #107 it is possible to connect floppy disk drives or DVD drives supporting rewritable media to the Pi and to expose them with s2p. These media usually offer different low-level formats (sector sizes, capacity), which are reported with READ FORMAT CAPACITIES.
In order to format these media, having the means for selecting one of the available formats is useful. A new formatting tool "s2pformat" shall list the available formats, so that the user can select which one to use for a subsequent FORMAT operation.