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/clock-util.c

@@ -26,10 +26,11 @@
 #include <sys/ioctl.h>
 #include <sys/time.h>
 
+#include "clock-util.h"
+#include "fd-util.h"
 #include "macro.h"
 #include "string-util.h"
 #include "util.h"
-#include "clock-util.h"
 
 int clock_get_hwclock(struct tm *tm) {
         _cleanup_close_ int fd = -1;