Bug#27894: mysqlbinlog formats timestamp wrong in comment

Date stamps lack zero padding, and so are meaningless.

Implement minimum-width zero padding for the percent-escape sequences
used in the logging code.


mysql-test/r/mysqlbinlog2.result:
  Include the long form, so we notice that it may be broken.  Replace
  away specific elements that vary from run to run.
mysql-test/t/mysqlbinlog2.test:
  Include the long form, so we notice that it may be broken.  Replace
  away specific elements that vary from run to run.
mysys/mf_iocache2.c:
  Implement minimum-width padding for printf-style percent-expressions.