Autoupdater using ioquake3's approach

[wildweasel]

I'd also consider adding a notice to any script error that reads, "Please ensure that you are running the latest available version from ZDoom.org" or similar. No need for an automatic checker.

[leileilol]

In OA's case FWIW, i'm not even going to take that autoupdate commit either (as it's upstream). Back in my day, people scorned autoupdates as "taking away rights from the end user", especially when Steam does it *shakes cane at cloud*

It would also cause inconsistencies with repositories downstream.

(also the really scary security bug is already averted by defaulting autodownload off, defaulting openal off and the released version not using renderer DLL modules anyway)