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/basic/process-util.c

@@ -820,13 +820,12 @@ int wait_for_terminate_with_timeout(pid_t pid, usec_t timeout) {
         for (;;) {
                 usec_t n;
                 siginfo_t status = {};
-                struct timespec ts;
 
                 n = now(CLOCK_MONOTONIC);
                 if (n >= until)
                         break;
 
-                r = RET_NERRNO(sigtimedwait(&mask, NULL, timespec_store(&ts, until - n)));
+                r = RET_NERRNO(sigtimedwait(&mask, NULL, TIMESPEC_STORE(until - n)));
                 /* Assuming we woke due to the child exiting. */
                 if (waitid(P_PID, pid, &status, WEXITED|WNOHANG) == 0) {
                         if (status.si_pid == pid) {