`fsync`: support app container jails

[mhils]

First off, thanks for the fantastic work on go-ios! :smiley: 🍰 Extending go-ios was super easy after figuring out what I needed with the debug proxy.

I've had a use case where I wanted to read/write files from/to a specific app container jail. Fortunately house_arrest already had everything I needed, and adding an --app switch to the fsync commands was straightforward.

1. Is accessing app jails for fsync something you folks are interested in supporting?
2. If yes, where do we want vendContainer to live? I've just copied it over from house_arrest.go to avoid a circular dependency, but I'm happy to do the legwork and move it into something like afc/house_arrest.go and have everything point there.