user name is calculated on function execution stage instead of parse stage


mysql-test/r/sp_notembedded.result:
  Bug#15298 SHOW GRANTS FOR CURRENT_USER: Incorrect output in DEFINER context
  test case
mysql-test/t/sp_notembedded.test:
  Bug#15298 SHOW GRANTS FOR CURRENT_USER: Incorrect output in DEFINER context
  test case
sql/mysql_priv.h:
  Bug#15298 SHOW GRANTS FOR CURRENT_USER: Incorrect output in DEFINER context
  new get_current_user(THD *thd, LEX_USER *user) function
sql/sql_acl.cc:
  Bug#15298 SHOW GRANTS FOR CURRENT_USER: Incorrect output in DEFINER context
  user name is calculated using get_current_user() function
sql/sql_parse.cc:
  Bug#15298 SHOW GRANTS FOR CURRENT_USER: Incorrect output in DEFINER context
  new get_current_user() function
  user name is calculated using get_current_user() function
sql/sql_yacc.yy:
  Bug#15298 SHOW GRANTS FOR CURRENT_USER: Incorrect output in DEFINER context
  empty LEX_USER struct for CURRENT USER, 
  user name is calculated on function execution stage