Warn about /doc (encountered by Colin Scott).
Improve multiconference support and add $Opt["include"].

$Opt["include"] lets us include more options files. The database
helpers understand it; so does PHP. Add multiconference support
to the database helpers via `-n CONFNAME` options. Document.
Prettier error message when a conference fails to load.
Add new createdb options, including --minimal and --batch.
Initial multiconferenceUrl support.