User registration module

This module allows users to register themselves. This plugin is aimed at (commercial) ZNC providers, not at individuals.

Besides simple registration, it contains two non-obvious features that are very important to me and maybe others.

First of all, it takes an optional argument. If this argument is provided, it is used as a salt to hash the username. It will also reveal a verification code field. The use case for this is that you can have your paywall spit out an url like `myawesomeznc.com/mods/global/register/?user=pepijn&code=deadbeef`

The second is that there is an `#ifdef` section that will add a default network if you configure with `CXXFLAGS="-DREGISTER_HOST=irc.example.com" ./configure`. This is because virtually all my users will connect to the same server and some people have trouble adding a network.

Of course, the module can also be used without any of those options, just allowing simple registrations.