Implement max concurrent connections (per IP) limit in web server, default: 10

We have this feature for the FTP, Mail, and Terminal servers (with no default
limit) but crazy abuse of my Synchronet web servers has now made this a
necessity for Vertrauen. A limit of 2 used to be considered reasonable long
ago. Today, 10 seems fair enough. Setting to 0 will impose no max-connections
per IP address (other than the total client limit per server).