ES_OPENCONTENT and ES_CLOSECONTENT now call Open and Close respectively,
as the old code did.

instead, leave all the management with the NANDContentLoader.

for file data (directly on the NAND), this opens the file on-demand and
returns the requested chunk when asked for it.
for on-the-fly decrypted WAD data, we just keep the decoded buffer in
memory, like we've done before - except that we don't give away any objects
we don't want to.

It's used by both the GUI to do things like install WADs and check up on
the system menu, in which case the global root should be used, and by
/dev/es, in which case the local one should.  The latter isn't
*terribly* useful today, since no contents will ever be installed in
temporary roots (although it's still relevant for data directories), but
converting the whole thing makes sense because then it will Just Work
once the entire NAND is synced.

Because it would have been a bit of work to split it up (but I can if
desired), this commit also contains some basic cleanup of
NANDContentLoader:

(1) The useless interface class INANDContentLoader is removed and the
    methods are changed to just return CNANDContentLoader (the only
    implementation);
(2) CNANDContentManager is changed to use unique_ptr and cleaned up a
    bit.

The cUIDsys constructor writes to <Wii user path>/sys/uid.sys.
This must not be done before Main.cpp sets the correct user paths.