New Behaviour of add command

Adding multiple files at the same time with `ipfs add` is basically broken. #3258 and #3300 basically propose different solutions.

#3258 adds and pins each entry specified on the command line individually without creating a common mfs-root, but it does not work with directories.

#3300 makes add behave more like tar and also adds the directories as given on the command line and then adds everything under a single pin and mfs-root

This issue is to work out the new behavior of the add command and the issues involved.

Start by comparing the proposed approaches in #3258 and #3300, focusing on how `ipfs add` handles multiple files, directories, pins, and the MFS root. Done means the competing behaviors and affected issues are resolved into an agreed specification for the new add behavior.

Written by the indexing model from the issue text.