added support for kill expr
fixed coredump in set @A := foo;
added testcase for user_var
added testcase for kill


sql/slave.cc:
  fd -> vio
sql/sql_class.cc:
  fd->vio, fixed coredump on set @A := foo;
sql/sql_class.h:
  fd -> vio
sql/sql_repl.cc:
  fd -> vio
sql/sql_yacc.yy:
  added support for kill expr - needed this for a clean test case of kill