Limit reader connections per IP address

[eagle]

nnrpd currently has no way of limiting connections per IP address other than using the custom auth hooks. In its daemon mode, it could in theory keep track of this and support throttling. It's probably not worth trying to support this when invoked via inetd, since at that point one could just use xinetd and its built-in support for things like this.

When started from innd, this is a bit harder. innd has some basic rate limiting stuff, but nothing for tracking number of simultaneous connections over time. It may be fine to say that if you want to use this feature, you need to have nnrpd be invoked separately, not run from innd.