Automatically zlib-compress large packets.
Make FileSearch less dumb in order to make me feel better and faciliate determinism.
Add g_HeadlessDeterminism, and stick it on for now.
Actually, make SD cards merely read only in g_HeadlessDeterminism mode.

The main use of SD cards in Dolphin is probably storing bulk data,
whether official (Rock Band) or unofficial (Project M), i.e. more like
the game disc than a save file.  This data needs to be available, e.g.,
to play Project M in that mode, and probably couldn't be treated like
the rest of the IPC data anyway because it's way too large (in both of
the aforementioned cases, it can be at least hundreds of megabytes,
although not all of that is going to be read during any particular
session).  It would be useful for Netplay and movies to detect whether
participants have the same SD card data, but that would need to be
separate.