Removed a few unnecessary includes. Made sector links use msecnodes so mobjs can now be linked as interior mobjs to multiple sectors which is necessary for portal cases where mobjs are half-way through portals.

git-svn-id: http://mancubus.net/svn/hosted/eternity/branches/portal-branch@2536 86ba831e-150c-0410-ab35-e06eaae15ed9
A bit of cleanup, and the beginnings of the secnode collection code for the portal clipping engine.

git-svn-id: http://mancubus.net/svn/hosted/eternity/branches/portal-branch@2542 86ba831e-150c-0410-ab35-e06eaae15ed9