Make Prepared_stmt_count a global status variable, accessible via
SHOW STATUS LIKE 'Prepared_stmt_count';.  Documentation should be
updated.


mysql-test/r/ps.result:
  Update result for bug#16365: Prepared Statements: DoS with too many
  open statements, according to bug#23159: prepared_stmt_count should be
  status variable.
mysql-test/t/ps.test:
  Update test case for bug#16365: Prepared Statements: DoS with too many
  open statements, according to bug#23159: prepared_stmt_count should be
  status variable.
sql/mysqld.cc:
  Add Prepared_stmt_count as global status variable.
sql/set_var.cc:
  Remove prepared_stmt_count as system variable.