When setting a blank expire string, meaning unlimited, curl would pass
TIME_T_MAX to getime_r() when creating the output, while on 64 bit
systems such a large value cannot be convetered to a tm struct making
curl to exit the loop with an error instead. It can't be converted
because the year it would represent doesn't fit in the 'int tm_year'
field!
Starting now, unlimited expiry is instead handled differently by using a
human readable expiry date spelled out as "unlimited" instead of trying
to use a distant actual date.
Test 1660 and 1915 have been updated to help verify this change.
Reported-by: Jonathan Cardoso
Fixes#7720Closes#7721
