Previously, a FORGET opcode could force a mount that was unused to be
forgotten.  This would cause the mounting filesystem to get into
inconsistent states: MultiZipFs and AutoUnionFs would believe the
mount is still live, but FilesystemConnector had already forgotten it.

The downside to this solution is that submount FileSystems will no
longer be unmounted if the root FileSystem is unmounted with
"fusermount -u".