time-util: add macros around timespec_store() that operates on compund literal allocated timespec struct

This way we can convert usec_t to timespec on-the-fly, without a buffer.

No actual behaviour change just some shortening of code.

src/nspawn/nspawn-stub-pid1.c

@@ -142,10 +142,8 @@ int stub_pid1(sd_id128_t uuid) {
 
                 if (quit_usec == USEC_INFINITY)
                         r = sigwaitinfo(&waitmask, &si);
-                else {
-                        struct timespec ts;
-                        r = sigtimedwait(&waitmask, &si, timespec_store(&ts, quit_usec - current_usec));
-                }
+                else
+                        r = sigtimedwait(&waitmask, &si, TIMESPEC_STORE(quit_usec - current_usec));
                 if (r < 0) {
                         if (errno == EINTR) /* strace -p attach can result in EINTR, let's handle this nicely. */
                                 continue;