Fix fsync with --app=bundleId

[zemanj]

Fixes #593.

According to libimobiledevice issue 193 from 2015 (in particular, this comment), the service com.apple.mobile.house_arrest doesn't support the VendContainer command anymore (unless the app was installed via XCode).
However, for apps that have UIFileSharingEnabled set to true in their information property list, the VendDocuments command can be used instead to access the app's /Documents directory.

This PR replaces VendContainer by VendDocuments to make this work.

Note that when using the go-iOS fsync command with the option --app=BUNDLE_ID set, the PATH passed with the option --path=PATH has to start with /Documents (in particular, / won't work).
For example, to use the fsync tree command for a specific app, one has to call ios fsync --app=BUNDLE_ID tree --path=/Documents.

[aluedeke]

@zemanj this code change cannot be merged like this, vendContainer and vendDocuments serve different purposes. One for xcode installed apps and one for apps sharing their documents, vendContainer doesn't work if an app doesn't share files. If you could come up with an proposal on how to support both modes we could merge this.