Read the default web_inactivity value from modopts.ini [web] inactivity value.

- this change is dependant on the updated load/modopts.js.
Cache the presence_lib and nodelist options in the bbs.mods object to speed
up the re-loading of this script from the BBS. This means that changes to the
presence_lib or modopts.ini will require the user log-off/back-on to experience
the changes (only for the nodelist module).