Refactor, wrap write() syscall in new dprint() function

Fixes the GCC warning for unchecked return value from write():

  ... ignoring return value of ‘write’, declared with attribute warn_unused_result

The new dprint() function wraps write() in a bounded retry-loop that
catches EINTR.  The length argument is optional computed with strlen()
if omitted (zero).

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/helpers.c

@@ -170,7 +170,7 @@ ssize_t cprintf(const char *fmt, ...)
 	va_end(ap);
 
 	for (i = 0; i < num_cons; i++)
-		(void)write(fds[i], buf, size);
+		dprint(fds[i], buf, size);
 
 	return size;
 }