mount-util: store mount flags in "todo" list in + handle submounts gracefully

mount-util: store mount flags in "todo" list in + handle submounts gracefully

This replaces the "todo" set with a "todo" hash map that stores the
mount flags we found. This makes an explicit call to get_mount_flags()
unncessary, since we have the flags handy right-away, and lowers our
work from O(n^2) to O(n). Nice!

The "done" set is also improved slightly: we'll use more modern ways to
allocate it, via set_ensure_consume(), and freeing-via-hash_ops.

Finally, failures on submount remounts are now handled gracefully,
there are just too many reasons why they might fail, given NFS, autofs,
FUSE which weird access controls, where even root might lack the privs
to do something.

Fixes: #16156