DERBY-6457: NetworkServerControl API breaks when username or password
contains non-ascii characters

Make NetworkServerControlImpl.writeLDString() write the length in
bytes instead of characters to match how it is read by
NetworkServerControlImpl.readLDString().

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1560311 13f79535-47bb-0310-9956-ffa450edef68