Provide the number of seconds since flush as a new status variable
named "Uptime_since_flush_status".
---
Override the post-flush value, as a second could pass between the two
statements.


mysql-test/r/information_schema.result:
  Test new status variable.
  ---
  Override the post-flush value, as a second could pass between the two
  statements.
mysql-test/t/information_schema.test:
  Test new status variable.
  ---
  Override the post-flush value, as a second could pass between the two
  statements.
sql/mysql_priv.h:
  Add variable to hold previous flush time.
sql/mysqld.cc:
  Add variable to hold previous flush time.  Insert name for 
  referring to it, and initialize the value.
sql/sql_show.cc:
  Yield value of elapsed time since flush when asked.
sql/structs.h:
  Add new show-flush-time to SHOW types enumeration.