(#1764908) core: Detect initial timer state from serialized data

[dtardon]

We keep a mark whether a single-shot timer was triggered in the caller's
variable initial. When such a timer elapses while we are
serializing/deserializing the inner state, we consider the timer
incorrectly as elapsed and don't trigger it later.

This patch exploits last_trigger timestamp that we already serialize,
hence we can eliminate the argument initial completely.

A reproducer for OnBootSec= timers:
cat >repro.c <<EOD
/*
* Compile: gcc repro.c -o repro
* Run: ./repro
*/
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <time.h>
#include <unistd.h>

    int main(int argc, char *argv[]) {
    	char command[1024];
    	int pause;

    	struct timespec now;

    	while (1) {
    		usleep(rand() % 200000); // prevent periodic repeats
           		clock_gettime(CLOCK_MONOTONIC, &now);
    		printf("%i\n", now.tv_sec);

    		system("rm -f $PWD/mark");
    		snprintf(command, 1024, "systemd-run --user --on-boot=%i --timer-property=AccuracySec=100ms "
    					"touch $PWD/mark", now.tv_sec + 1);
    		system(command);
    		system("systemctl --user list-timers");
    		pause = (1000000000 - now.tv_nsec)/1000 - 70000; // fiddle to hit the middle of reloading
    		usleep(pause > 0 ? pause : 0);
    		system("systemctl --user daemon-reload");
    		sync();
    		sleep(2);
    		if (open("./mark", 0) < 0)
    			if (errno == ENOENT) {
    				printf("mark file does not exist\n");
    				break;
    			}
    	}

    	return 0;
    }
    EOD

(cherry picked from commit aa1f95d)

Resolves: #1764908

[msekletar]

LGTM