This makes the device ID assigning code common to all backends, by
moving it to AddDevice() instead of copy-pasting or replicating
the logic in the backends.
Also, to prepare for hotplugging, instead of relying on a name usage
count, the new ID assigning system always starts from ID 0 and tries
to assign the first ID that is not used.
Previously, MacOpenFile only overrode anything on OS X; otherwise it was
just a useless method, which is presumably why it wasn't marked override
in the first place. Address this more sanely by wrapping it in #ifdef
__APPLE__.
