update rcube_plugin_api::exec_hook() to support new hooks that have been added mid-run

This pull request fixes 2 things

1) fix tracking of the active_hook state

2) during the hook execution loop check for newly added hooks mid-run

Example: Plugin A dynamically loads plugin during exec_hook:startup. Without this fix the startup hook of plugin B won't get executed anymore. (In this case kolab_auth plugin during kolab_auth_role_plugins execution)

Additional information to the while loop: 
* foreach won't see react on new added hooks
* using array pointers with while(next()) will not work cause adding new hooks will reset the current array pointer position.

I tried to keep it lightweight and hope this check doesn't affect too much of the performance.

What do yo think?