some db comparison code cleupup
removed compiler warnings


mysql-test/r/lowercase_table.result:
  test for BUG#3251
mysql-test/t/lowercase_table.test:
  test for BUG#3251
sql/item.cc:
  simple strcmp can be used for db name comparison
sql/set_var.h:
  changed initialisation order to satisfy compiler
sql/sql_cache.cc:
  simple strcmp can be used for db name comparison
sql/sql_db.cc:
  better use special cherset for files
sql/sql_parse.cc:
  simple strcmp can be used for db name comparison
  aliases should be compared with my_strcasecmp
sql/sql_select.cc:
  aliases should be compared with my_strcasecmp