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)

Also updated tests to reflect new show grants functionality.

The output is not completely correct due to recursive role grants not
being completly implemented. However, this will help with testing the
implementation of set role with recursive grants.

The function now performs a DEPTH FIRST SEARCH on the role graph.

At various key points: on_start, on_open, on_cycle, on_finish,
the function calls one of the corresponding functions passed as parameters.

function.

The function will be used to help print roles privileges recursively.

function.

The function will be used to help print roles privileges recursively.

function.

The function will be used to help print roles privileges recursively.

The privileges are not correctly updated via grant commands yet.

The bug was caused by not renaming the role if it was previously
modified by the handle_grant_struct(ROLE_ACL,...) call.

The same function used find_acl_role and would search for the already
renamed role when it handled ROLES_MAPPINGS_HASH. This caused it to not rename
the role/user correctly.

not support roles

Also fixed issue with drop role not clearing internal memory entry
for that role. The issue was due to a condition introduced in handle_grant_data

Updated testsuite to also check the possible error conditions.

Revoke privilege on *.* to role@''; also works