avoid deprecated (by linux) alarm syscall; use setitimer instead

src/unistd/alarm.c

 #include <unistd.h>
+#include <sys/time.h>
 #include "syscall.h"
 
 unsigned alarm(unsigned seconds)
 {
-	return syscall(SYS_alarm, seconds);
+	struct itimerval it = { .it_value.tv_sec = seconds };
+	__syscall(SYS_setitimer, ITIMER_REAL, &it, &it);
+	return it.it_value.tv_sec + !!it.it_value.tv_usec;
 }