BUGFIX: Signal when a duplicate account is being loaded

If a duplicate account is being created or loaded then the functions
return -1. The higher code can take it from there.
Implemented saving of user accounts (read more)

To modify an existing user account, change the in memory copy and then
call the function SaveUserAccount(uid), where uid is the index of the
modified account. If the save function is not called immediately, then the
behaviour of the server is undefined.
Added Unloading and Deletion of user accounts