This worklog adds a system variable binlog_expire_logs_seconds in
addition to the existing expire_logs_days variable.
The ultimate requirement is that the user can set expire periods
smaller than one day by providing another extra variable.
The new variable binlog_expire_logs_seconds, will be set in those cases
where the expire period is not a integer multiple of days
like 1 day 2 hours and 32 minute.

@ sql/binlog.cc

  The purge time now also considers the value of
  binlog_expire_logs_seconds.

@ sql/mysqld.h

  Declared the new variable binlog_expire_logs_seconds

@ sql/mysqld.cc

  The purge time now also considers the value of
  binlog_expire_logs_seconds.

@ sql/sys_vars.cc

  Added the description for binlog_expire_logs_seconds, and
  modified the description for expire_logs_days.

Added test case to test the new variable:

- binlog_expire_logs_seconds: Ensure that when any of the two expire
  variable, expire_logs_days or binlog_expire_logs_seconds is set
  the purge happens on server restart and in cases where a force
  rotation of binlog happens.

Extended the existing test cases:
- expire_logs_days_basic: Added code to test the system variable
  binlog_expire_logs_seconds.