tzcode: Fix early tz change detection

[des]

Summary

Assume tzdata is not fresh if last_checked is zero, as comparing the
current time to last_checked less than __tz_change_interval after boot
may produce a false negative.

While here, invert the return value from tzdata_is_fresh() to better
match its new name (it was previously called recheck_tzdata(), so zero
for fresh and non-zero for stale made sense, but it doesn't now).

PR: 269207
MFC after: 3 days

[des]

initializing last_checked to TIME_T_MIN did not work, presumably because the subtraction overflowed, so explicitly check for zero instead.

[des]

reorder