1. Introduced a cfs_offset_t type that designates file offsets. unsigned int
was insufficient on several platforms with non-volatile storage that exceed
the capacity of unsigned int.
2. Switched cfs_seek signature to POSIX style with a "whence" parameter.
- Productive embedded targets don't use the posix based variant anyway.
- For testing purposes the prefix is a hurdle as the documentation doesn't mention it ;-))
- The prefix is incompatible with posix implementations lacking directory support (like the C64).
This change means that one needs to select _ONE_ cfs implementation at Contiki library link time. But this doesn't appear to be an issue as all platforms have their "favorite" implementation anyway.
