Fix #166: pmbootstrap shutdown: umount deep folder levels first

Created by: ollieparanoid

• 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)