mainly to avoid the pattern of
* get username/hostname/rolename
* optionally find the corresponding ACL_USER and ACL_ROLE
* allocate memory, concatenate username/hostname/rolename
* call a function passing only this memory as an argument
** use concatenated username/etc to find ACL_USER and ACL_ROLE again
** do something
* free the object

Also to undo push_dynamic we use pop_dynamic now,
not a linear search/scan through the dynamic array.

as a bonus, role@ is now an invalid way to refer to a role.

(and an assert in myisam to catch these bugs easier in the future)
update tests/results

  CREATE ROLE xxx WITH ADMIN yyy
  GRANT xxx TO yyy WITH ADMIN OPTION
  REVOKE ADMIN OPTION FOR xxx FROM yyy

sql/sp.cc:
  don't split "user@host" string in db_load_routine, because the caller needs to
  generate it from user and host. instead pass user and host directly into db_load_routine
sql/sql_parse.cc:
  1. REVOKE ALL doesn't need invoker.
  2. make sp_process_definer() reusable
sql/sql_trigger.cc:
  don't duplicate the code from sp_process_definer(), reuse it
sql/sql_view.cc:
  don't duplicate the code from sp_process_definer(), reuse it

* work as documented, use CURRENT_USER()
* move the check for ER_PASSWORD_ANONYMOUS_USER where it can actually work

than an empty host '' is the same as any-host wildcard '%'.

Replace '' with '%' in the parser (for GRANT ... foo@'') and when loading grant tables.

Side effect: one cannot have foo@'' and foo@'%' both at the same time
(but one can have foo@'%' and foo@'%%')

Also fixed possible memory exploit by repeteadly calling:
GRANT role to user; where role was already granted to user.

should deal with users, while role related functions should deal
with roles.

TODO:

Privilege checks are not done upon executing the command.

Fixed segmentation fault caused in traverse_role_graph by previous commit

was halted on a cycle detect. Now the to_clear array is populated during
the open event and not the close event.

because parser might modify the lex->user (e.g. set lex->user-password).
switch to use LEX_STRING current_user string, and also change other similar constants
to be LEX_STRING's for consistency.

mysql-test/r/acl_roles_show_grants.result:
  one can do SHOW GRANTS for himself
mysql-test/t/acl_roles_set_role-table-column-priv.test:
  correct error message
mysql-test/t/acl_roles_show_grants.test:
  one can SHOW GRANTS for himself
sql/sql_acl.cc:
  bugfixing:
  * don't assign with && - it can shortcut and the second assignment won't be executed
  * correct the test in check_grant_all_columns() - want_access should not be modified
  *
sql/sql_cmd.h.OTHER:
  add new commands at the end
sql/sql_db.cc:
  don't call acl_get() if all privileges are already satisfied
  (crashes when run with --skip-grants, because acl data stuctures aren't initialized)
sql/sql_parse.cc:
  * test for current_user in get_current_user()
  * map explicitly specified user@host to current_user

The command only currenty affects in memory data structures. Writing to
the roles_mapping table needs to be implemented.

The warnings present in the set_role_routine-simple testcase will
be removed when reworking the grant privilege to call.

to the hash_columns' original elements (GRANT_COLUMN)