core: Detect initial timer state from serialized data

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 aa1f95d2)