Use forced close of socket to make mysqld shutdown faster when used under valgrind


mysql-test/mysql-test-run.sh:
  Added --skip-bdb for valgrind
mysql-test/r/user_var.result:
  Extended test for user variables
mysql-test/t/user_var.test:
  Extended test for user variables
sql/item_func.cc:
  Optimized code for setting user variables with := and fixed some bugs in old code
sql/item_func.h:
  Optimized code for setting user variables
sql/log.cc:
  Fixed comments
sql/mysqld.cc:
  Use forced close of socket to make mysqld shutdown faster when used under valgrind
sql/sql_class.h:
  Optimized code for setting user variables