openat: additional test cases

 - Test jump-symlink with destination below dfd (but which is rejected
   anyway because it's a jump-symlink).
 - Test opening . and .. explicitly (and adjust test to only try to
   read non-directories).
 - Test things that are actually within the dfd, but get there via
   a .. hop along the way.
openat: emit failcount from selftest
execveat: allow fexecve() of O_PATH files
execveat: use kasprintf rather than error-prone calculations
execveat: self-test names bigger than PATH_MAX
execveat: selftest that O_PATH files get their execute permission checked
execveat: update selftest now O_PATH files executable
execveat: test fexecve() against deleted file with O_PATH fd still open
execveat: add a better long pathname test