Merge branch 'jk/sanity' into maint

The tests that wanted to see that file becomes unreadable after running "chmod a-r file", and the tests that wanted to make sure it is not run as root, we used "can we write into the / directory?" as a cheap substitute, but on some platforms that is not a good heuristics. The tests and their prerequisites have been updated to check what they really require. * jk/sanity: test-lib.sh: set prerequisite SANITY by testing what we really need tests: correct misuses of POSIXPERM t/lib-httpd: switch SANITY check for NOT_ROOT