Unmount Mac .sparseimage in case of errors while mounted.

DarwinManifest.package_finish() creates and mounts a .sparseimage file,
populating it with the desired package contents before creating a .dmg from
it. However, errors while it's mounted would formerly bypass the 'hdiutil
detach' command that unmounts it, leaving the host in a confused state that
seemed to prevent subsequent retry attempts from succeeding.
Also add code to try to diagnose/bypass mysterious SetFile -a V filename
errors, so far without much luck.