Updated MySQL time handling code to react correctly on UTC leap second additions.
MySQL functions that return the OS current time, like e.g. CURDATE(), NOW() etc
will return :59:59 instead of :59:60 or 59:61.
As a result the reader will receive :59:59 for 2 or 3 consecutive seconds 
during the leap second.
This fix will not affect the values returned by UNIX_TIMESTAMP() for leap seconds.
But note that when converting the value returned by UNIX_TIMESTAMP() to broken 
down time the correction of leap seconds will still be applied.
Note that this fix will make a difference *only* if the OS is specially configured
to return leap seconds from the OS time calls or when using a MySQL time zone 
defintion that has leap seconds.
Even after this change date/time literals (or other broken down time 
representations) with leap seconds (ending on :59:60 or 59:61) will still be 
considered illegal and discarded by the server with an error or 
a warning depending on the sql mode.
Added a test case to demonstrate the effect of the fix.

mysql-test/r/timezone3.result:
  Bug #39920: test case
mysql-test/std_data/Moscow_leap:
  Bug #39920: updated the Moscow time zone to Dr. Olson's tzdata 2008i 
  to accomodate for the 2008 leap second
mysql-test/t/timezone3.test:
  Bug #39920: test case
sql/tztime.cc:
  Bug #39920: adjust leap seconds (:60 or :61) to :59
sql/tztime.h:
  Bug #39920: adjust leap seconds (:60 or :61) to :59