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.
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.