Bug #15684: @@version_* are not all SELECTable

  Added the appropriate information as read-only system variables, and
  also removed some special-case handling of @@version along the way.

  @@version_bdb was added, but isn't included in the test because it
  depends on the presence of BDB.


mysql-test/r/variables.result:
  Update results
mysql-test/t/variables.test:
  Fix error from setting @@version, reset sql_select_limit to avoid confusion
  in later tests, and add new regression tests.
sql/item_func.cc:
  Remove special-case handling of VERSION, it's just a read-only server
  variable like the others now.
sql/set_var.cc:
  Add system_time_zone and various version-related system variables