clock-util: be more tolerant in parsing /etc/adjtime

clock-util: be more tolerant in parsing /etc/adjtime

As we default to "hardware clock is in UTC" if /etc/adjtime is not present, it
also makes sense to have that default if /etc/adjtime contains only one or two
lines.

Drop the "gibberish" test case, as this was just EIO because of not containing
three lines, which is already contained in other tests. clock_is_localtime()
never actually validated the format of the first two lines, and there is little
point in doing that.

This addresses the reading half of issue #2638.