IOS/FS: Add a scoped FD class to make it harder to leak FDs

[leoetlino]

This changes FileSystemProxy::Open to return a file descriptor wrapper
that will ensure the FD is closed when it goes out of scope.

By using such a wrapper we make it more difficult to forget to close
file descriptors.

This fixes a leak in ReadBootContent. I should have added such a class
from the beginning... In practice, I don't think this would have caused
any obvious issue because ReadBootContent is only called after an IOS
relaunch -- which clears all FDs -- and most titles do not get close
to the FD limit.

[leoetlino]

Like you have to remember to call it to properly close the object

No, Release does the opposite: it's like unique_ptr<T>::release, it relinquishes control of the file descriptor so that the caller is now responsible for closing the file (with unique_ptr you'd need to delete the pointer yourself after calling release).

It is necessary when handles are directly exposed to the emulated console, because what is responsible for closing the handle is emulated code, not Dolphin.

[leoetlino]

All comments addressed, I think.