[IMP] core: require install mode for db bootstrap

The registry loading system should not alter databases unless it is
asked to do so, by a module installation or update instruction.
This property should hold true as well for database bootstrap, and this
is what this commit changes..

In order to avoid any behavior change for command-line users, an
implicit `-i base` is assumed when starting the server from the
command-line with `-d <db>`, causing the db boostrap to happen if the
database did not exist yet.