Major cleanup of fastbufs:

Major cleanup of fastbufs:

  o  Split generic fastbuf from low-level routines. `struct fastbuf'
     no longer contains low-level data like `fd' or `is_temp_file'.
  o  Introduced safe type casting macros to avoid programming errors.
  o  `struct fastbuf' is no longer freed by the high-level code.
  o  Documented behaviour of bflush() between reads and writes.
  o  Redefined semantics of fastbuf->pos: it now corresponds to `bstop'
     instead of `buffer', hence it always coincides with real file
     position, making `fdpos' unnecessary.