Refactor/Redo $Home directory fix for the transfer plugin

[turetske]

-- InitConfig went from not being able to fail to calling os.Exit directly. A library should never, ever exit the process. That's the job of the upper level tool. Remove all os.Exit accesses in InitConfig
-- initConfigDir is now public, meaning there's an additional step to setup the configuration. That's indicative of something else gone wrong. We should make this private again and update the tests to call InitConfig correctly
-- There's now been multiple regressions around handling of $HOME in the last year. Let's establish a unit test to get the right behavior in place ($HOME unset, $HOME set but missing, etc) to make sure we understand what we want.