Move Z_Init out of i_main (there's nothing system specific about it, and it no longer has to be called that early either). Re-added some old-school nostalgia too.

git-svn-id: http://mancubus.net/svn/hosted/eternity/trunk@2467 86ba831e-150c-0410-ab35-e06eaae15ed9
Move nostalgia message to d_main.cpp and have it use startupmsg like the others.

git-svn-id: http://mancubus.net/svn/hosted/eternity/trunk@2468 86ba831e-150c-0410-ab35-e06eaae15ed9