* Add: a comprehensive downsides FAQ with mitigations and practical
priorities.
* Document: mergerfs-webui and link it from setup and configuration
guides.
* Replace: obsolete mergerfs.fsck references with fsck.mergerfs
guidance.
Run in an elevated credential mode (root) and let the kernel manage
entitlements. Will result in slightly different behavior but should
not be noticed by most.
Also have special error handling for when branches are invalid and
ENOENT would be returned for getattr and readdir so users understand
what is going on and the runtime interface can still be used to fix
the problem.
