I've replaced all instances in the code of `os.path.abspath`
with `os.path.realpath`, as this does the same as `abspath`
plus resolving symlinks.
See also: https://stackoverflow.com/a/40311142
* Refactored `umount_all` to get the list of folders to be umounted from
`umount_all_list`, so we can test that function in a test case.
* Adjusted `umount_all_list` to return the deep folder levels first
* Added a testcase for that
* Remove redundant calls to `umount_all()` (which were from a time before
`pmbootstrap` was released, in which failing commands did not cause
`pmbootstrap` to raise an exception)
Oliver Smith