readutmp: Make it easier to filter for/against the boot-time entry. * lib/readutmp.h (READ_UTMP_BOOT_TIME, READ_UTMP_NO_BOOT_TIME): New enum items. * lib/readutmp.c (desirable_utmp_entry, read_utmp_from_systemd): Implement them. (read_utmp): If no entries can match the given options, return immediately.