Tweak install steps to be on the POST rather than GET. It makes more sense architectu...

Tweak install steps to be on the POST rather than GET. It makes more sense architecturally. It also gives us one last chance for them to say no (in case they clicked "I'm bored already" but changed their minds). Check final step of installer for fatals, and if everything's good, suppress the back button. It's just going to confuse people. And the opposite, suppress continue on fatals...no need pretending things went dandy if they didn't. It's basically now impossible to re-run 'page=Install' after it completes. This is probably a good thing as it throws errors. Coincidentally this also fixes bug 26947.