Allow empty key list in USE|IGNORE|FORCE INDEX()


mysql-test/r/myisam.result:
  New explain format
mysql-test/t/myisam.test:
  test of index()
mysys/my_handler.c:
  Updated ha_key_cmp() to be as latest _mi_key_cmp() in 4.0
sql/sql_lex.cc:
  Post merge fix
sql/sql_lex.h:
  Post merge fix
sql/sql_parse.cc:
  Post merge fix
sql/sql_yacc.yy:
  Post merge fix for FORCE INDEX ()
  Allow empty key list in USE|IGNORE|FORCE INDEX()