two versions of KILL implementation


include/mysql_com.h:
  ONLY_KILL_QUERY flag definition
sql/lex.h:
  CONNECTION keyword added
sql/mysql_priv.h:
  kill_one_thread function extended
sql/sql_class.h:
  only_kill_query flag added to the THD structure
sql/sql_parse.cc:
  handling of KILL QUERY feature added
sql/sql_yacc.yy:
  KILL syntax extended with CONNECTION an QUERY options