util-lib: split out fd-related operations into fd-util.[ch]

There are more than enough to deserve their own .c file, hence move them
over.

src/basic/socket-label.c

@@ -19,18 +19,19 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <string.h>
-#include <unistd.h>
 #include <errno.h>
-#include <sys/stat.h>
 #include <stddef.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
 
+#include "fd-util.h"
 #include "macro.h"
-#include "util.h"
-#include "mkdir.h"
 #include "missing.h"
+#include "mkdir.h"
 #include "selinux-util.h"
 #include "socket-util.h"
+#include "util.h"
 
 int socket_address_listen(
                 const SocketAddress *a,