Use fchmodat and fchownat.
Remove shell script-based test for touch.
Use openat/fstat for getattr.
Add test to stat by path while a file is already open.
Add tests for mkdir/rmdir.
Add tests for symlink manipulation.
Add tests for rename.
Add test for unlink.
Use page_buffer as in-memory backing store for file contents.
Add tests for access.

This changes the behavior to reflect open's access checking more accurately:
If a file is currently open (with flags O_CREAT and not O_APPEND), it can be
opened for reading.
Fix leak of /dev/null file descriptors.