Extract the FUSE implementation from the main sdcard.c file.

sdcard.c is a *really* big file. This makes it hard to do things like
improving priv dropping or adding more sandboxing. Extract all
FUSE-related code to a separate unit, fuse.{h|c}, which exports only
two functions. Convert the rest of sdcard.c to C++ as sdcard.cpp.
fuse.c is kept as C (at least for now) since interacting with the FUSE
API is realistically easier from C.

Bug: 30110940

Change-Id: I188bfdc21c184742117e07539adb09090d4d747c