- In function 'handle_grant_struct' when searching the memory structures for an 
   entry to modify, convert all entries here host.hostname is NULL to "" and compare that 
   with the host passed in argument "user_from".
 - A user created with hostname "" is stored in "mysql.user" table as host="" but when loaded into 
   memory it'll be stored as host.hostname NULL. Specifiying "" as hostname means
   that "any host" can connect. Thus is's correct to turn on allow_all_hosts
   when such a user is found. 
 - Review and fix other places where host.hostname may be NULL.


mysql-test/init_db.sql:
  Rename: BitKeeper/deleted/.del-init_db.sql~af2dfeabaa348dd7 -> mysql-test/init_db.sql
mysql-test/r/grant.result:
  Add test cases for users with hostname ""
mysql-test/t/grant.test:
  Add test cases for users with hostname ""
   - Test create, grant and drop as well as connecting as the user with hostname "".
sql/sql_acl.cc:
  Convert places where host.hostname is NULL to "" before using it.