NetworkServerControl API breaks when username or password contains non-ascii characters

If you pass a non-ascii username/password combination to a NetworkServerControl API command, such as

java -jar derbynet.jar shutdown -user Bjørn -password Dæhlie

it will hang forever.

The reason is that NetworkServerControlImpl.writeLDString() writes the length of the string in characters, whereas NetworkServerControlImpl.readLDString() expects the length to be in number of bytes.