Rewrote to use pread/pwrite() and to avoid messing up with internals

Rewrote to use pread/pwrite() and to avoid messing up with internals
of the fb-file module. It should work with file descriptors shared between
processes (no need to re-open the file after fork()) and should be also
somewhat faster.

Basic functions already work, it remains to create some utilities
and implement compression.

make obj/lib/bucket-t for a small test program.