[installer][host] Get stdout/err from dd on OSX
[installer][host] fix a bug where the installer will crash on OSX when there is a mounted unitialised disk (this messes up the indices for nixDevice)
[installer][host] prepare worker to write image concurrently and setup image-write routine for osx
[installer][host] fix offline extraction. Needs verfification on Linux and Windows. DownloadProgress:27+  has to sanitize the filelocation from the URL
[installer][host] diskutil unmountDisk without root? Interesting! Please test on other OSX machines!
[installer][host] Make sure that we use the correct image name after extraction
[installer][host] ooops, here's the rest of the offline-build fix.