Add 'first_node' and 'last_node' properties to JS bbs object

This allows scripts (e.g. login.js) to have custom behavior (e.g. shortening
the maximum inactivity timeout) based on how close the current node is to the
configured last node number. There may be other uses too, but for the vast
majority of Synchronet systems, first_node will always be 1 and last_node the
same as system.nodes/lastnode. Ugh, inconsistent naming. :-(