Game saves breaks replays

[clementgallet]

We should prevent games from saving. This should be done quite easily by hooking file IO functions and preventing file writing.

[clementgallet]

Writing into regular files is not prevented (be67570 and e9abac2).

[clementgallet]

Add more fileio hooks (cf72cfb), but still problems with Braid, probably because the savefile is modified in another way (deleted or truncated). So we must deal with this. A simple idea is to switch opening those files to readonly so we should have explicit error messages.

[clementgallet]

Change of strategy: now we make a copy of any file considered as a save file, and open the copy (66563ec). This is a much easier method and seems to work great.