Implement serialisation of users.•••This allows for various things which will be coming in the future.
e.g. Transferring users to another server on upgrade.
Remove the original line parameter of On{Pre,Post}Command.•••In the brave new world of message tags and alternate wire formats
this is no longer something that is appropriate to expose.
In reality it was only ever used by m_alias which now reconstitutes
the command name and parameters into a RFC 1459-style message for
whatever it needs to do.