Add new options -bcCklL and fix some porting problems.

Add new options -bcCklL and fix some porting problems.
Remove options -dp.  Do not read output files.

(long_opts, usage, main, wipefile): Adjust to new options.
("human.h", "quotearg.h"): New includes.
(struct Options): New members contents, links, n_iterations.
Remove allow_devices, remove_file.  Change n_iterations to size_t.
All uses changed.
(output_block_size): New var.
(usage): Declare __noreturn__ attribute.
(fdatasync): Define to -1 if not present, since we need to invoke both
fdatasync and fsync if both are present.  All invokers of fdatasync
now try fdatasync, then fsync.
(MIXIN): New macro.
(isaac_seed): Use it to mix in values.  Add uid, gid to mix.
Don't use gettimeofday, as it has too many porting problems.
(isaac_seedfd): Remove, since we no longer read the output files.
(sizefd): Remove; we now determine size by writing sequentially.
(dopass, wipename, wipefile, main): Clean up error messages.
(dopass): Keep track of offset relative to start of file, not
end, since we may not know how large the file is.  If size is
negative, write until we fall off the end of the file.
(wipefd): Do not read output file.
Return 0 if successful, -1 if not; do not make a special case for
non-regular files, since our callers have that info now.
(wipename): Now static.  Return errno if error.
(main): "-" now stands for standard output.
Do not shred append-only standard output.
(wipefile): Do not grant read permission to file when wiping it.
Use symbolic permission (S_IWUSR), not octal.
From Paul Eggert.