Added new logic to ACL system:

1) If GRANT OPTION (not mysql db):
   Ok to update existing user, but not password.
   Not allowed to make a new user.

2) If UPDATE_ACL to mysql DB:
   Ok to update current user, but not make a new one.

3) If INSERT_ACL to mysql DB:
   Ok to add a new user, but not modify existing.

4) If GRANT OPTION to mysql DB:
   All modifications OK.


mysql-test/r/grant2.result:
  Added more ACL tests and fixed results in some old tests.
mysql-test/t/grant2.test:
  Added more ACL tests and fixed results in some old tests.
sql/sql_acl.h:
  Made check_acl_user() visible to sql_parse.cc
sql/sql_parse.cc:
  Added new logic to ACL system:
  
  1) If GRANT OPTION (not mysql db):
     Ok to update existing user, but not password.
     Not allowed to make a new user.
  
  2) If UPDATE_ACL to mysql DB:
     Ok to update current user, but not make a new one.
  
  3) If INSERT_ACL to mysql DB:
     Ok to add a new user, but not modify existing.
  
  4) If GRANT OPTION to mysql DB:
     All modifications OK.