fat: add 'cat' subcommand to dump a named file to stdout
When reading a file, clip the length to 0

This avoids us reading off the end of a file, and doing nonsensical
things like 'Cstruct.sub s (-ve offset)...' which leads to trouble
Correct the calculation of the 'end sector' and 'preceeding bytes'
Add SectorMap.to_string for better debugging