The to_detach target_ops method implementations are currently expected
to work on current_inferior/inferior_ptid. In order to make things more
explicit, and remove some "shadow" parameter passing through globals,
this patch adds an "inferior" parameter to to_detach. Implementations
will be expected to use this instead of relying on the global. However,
to keep things simple, this patch only does the minimum that is
necessary to add the parameter. The following patch gives an example of
how one such implementation would be adapted. If the approach is deemed
good, we can then look into adapting more implementations. Until then,
they'll continue to work as they do currently.