Bug #39920: MySQL cannot deal with Leap Second expression in string literal.

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.

Original changesets:
> revision-id: kgeorge@mysql.com-20081201141835-rg8nnnadujj5wl9f
> parent: gshchepa@mysql.com-20081114172557-xh0jlzwal8ze3cy6
> committer: Georgi Kodinov <kgeorge@mysql.com>
> branch nick: B39920-5.0-bugteam
> timestamp: Mon 2008-12-01 16:18:35 +0200

> revision-id: kgeorge@mysql.com-20081201154106-c310zzy5or043rqa
> parent: kgeorge@mysql.com-20081201145656-6kjq91oga5nxbbob
> committer: Georgi Kodinov <kgeorge@mysql.com>
> branch nick: B39920-merge-5.0-bugteam
> timestamp: Mon 2008-12-01 17:41:06 +0200