Rewrite to avoid some unnecessary casts, macros, literals.

Rewrite to avoid some unnecessary casts, macros, literals.
(DEFAULT_PASSES, VERBOSE_UPDATE): Now constants, not macros.
(SECTOR_SIZE, SECTOR_MASK): New constants.
(fillpattern, dopass, do_wipefd, main): Remove unnecessary casts,
and use the SECTOR_* constants when applicable.  Check for size <
0 rather than size == -1, since negative-size files are a sign of
trouble anyway.

(dopass): Assume a continuable error if EIO even if the current
position is not a multiple of 512.