Completely rewritten buffer management of the sorter.

Completely rewritten buffer management of the sorter.

The big_buf is no longer split in a fixed way. Every sorter module now
decides on its own how to subdivide the buffer. This leads to better
utilization of the buffer space (e.g., when not using a radix-sorter)
and to the possibility to adjust workspace used for merging via the
(new) SORT_UNIFY_WORKSPACE macro.

s-fixint is now applied only if no workspace is required, but this should
cover all known uses of unification on fixed-size records anyway.