- Avoid overflow in sec_since_epoch by shifting the time
   back 2 days for times close to the maximum range of my_time_t
 - Improve comment about why we need my_time_t
 - Patch will also fix timezone2.test


include/my_time.h:
  Improve comment for my_time_t, especially that we require
  it to be at least a 32bit unsigned type
sql/tztime.cc:
  - Add assert in sec_since_epoch to guard it against broken down time
  values that would cause overflow in my_time_t on systems with 32 bit
  my_time_t
  - Avoid overflow in sec_since_epoch by "shifting" the broken down
  time back 2 days for times that are near the max value.c