In shred, check for errors from fdatasync more carefully. If

In shred, check for errors from fdatasync more carefully.  If
fdatasync fails with errno==EINVAL, it means this implementation
does not support synchronized I/O for this file.  Do not report
this as an error, as (for example) AIX 5.2 fdatasync reports it
for raw disk devices.  Problem reported by Albert Chin in
<http://mail.gnu.org/archive/html/bug-gnu-utils/2004-05/msg00028.html>.

Check for write errors, though: the old code ignored them.
Improve error checking in a few other cases, too (e.g., close of a
directory).

Also, change several 'int' values to 'bool', so that the error
checking is a bit clearer.  Similarly, change unsigned values
to size_t where appropriate.

* src/shred.c: Include "dirname.h".
(datasync) [!HAVE_FDATASYNC]: Remove.
(dosync): New function.
(dopass): Use it.  Return 1 on write error, -1 on other error.
All callers changed.  Report write error if dosync does.
(do_wipefd, wipefd, wipename, wipefile): Return bool (true/false),
not int (0/-1).  All callers changed.  Return false if there's a
write error.
(incname): Return bool (true/false), not int (0/1).  Accept
size_t length, not unsigned.  All callers changed.  Do not
bother checking for non-digits; it can't happen.  Replace
recursion with iteration.
(wipename): Use dir_name, base_name, etc. instead of assuming
Unix file names.  Use size_t for length, not unsigned.
Report error if unlink or close fails.
(wipename, main): Use bool for booleans.

(names): Use only digits and uppercase letters, for greater
portability.