accounts: streamline API

- Manager.Accounts no longer returns an error.
- Manager methods take Account instead of common.Address.
- All uses of Account with unkeyed fields are converted.

cmd/gethrpctest/main.go

@@ -116,7 +116,7 @@ func MakeSystemNode(keydir string, privkey string, test *tests.BlockTest) (*node
 		if err != nil {
 			return nil, err
 		}
-		if err := accman.Unlock(a.Address, ""); err != nil {
+		if err := accman.Unlock(a, ""); err != nil {
 			return nil, err
 		}
 	}