plugins: when loading active ones, loop until no more proxy plugins are added
During the loading of the active plugins they are also initialized (done at
startup). As a result, these plugins could be pluxys and make more plugins
available, some of which may be active as well.
Because of this the loop has to be restarted if pluxies become
available to also load active plugins that depend on the pluxy.
The loop is only restarted at the end so only nested pluxys could possibly
cause the loop to be run more than twice.