When sandboxing is enabled, we could not write any stats to disk.
check_or_create_data_subdir("stats"), which prepares the private stats
directory, calls check_private_dir(), which also opens and not just stats() the
directory.  Therefore, we need to also allow open() for the stats dir in our
sandboxing setup.

If we manually remove fallbacks in C by adding '/*' and '*/' on separate
lines, stem still parses them as being present, because it only looks at
the start of a line.

Add a comment to this effect in the generated source code.

Comment-only change

No behavioural change.
Also clarify some comments.