Split 'BufFile' routines out of fd.c into a new module, buffile.c. Extend

Split 'BufFile' routines out of fd.c into a new module, buffile.c.  Extend
BufFile so that it handles multi-segment temporary files transparently.
This allows sorts and hashes to work with data exceeding 2Gig (or whatever
the local limit on file size is).  Change psort.c to use relative seeks
instead of absolute seeks for backwards scanning, so that it won't fail
when the data volume exceeds 2Gig.