FUSE mounts over a non-empty folder without complaint, so files already
in the My files folder were hidden under the mount: not uploaded, and
back only once the service stopped, so they looked deleted (#67, B277).
- pdfs-core: new `mountpoint` module. `not_empty` lists what is in a
folder (hidden entries count); `mounted_fs_type` moves here from the
CLI so the daemon can use it. `MountFailure` gains an optional
`not_empty` field, which older readers ignore.
- pdfs-fuse: `mount_with` checks the folder before the root fetch and
before any worker starts. A dead mount is cleared first; one of our
own live mounts is left to `spawn_session`, as before.
- pdfs-cli: `cmd_daemon` records the folder in the failure record. The
refusal backs off and retries like any mount failure, so emptying the
folder is enough.
- pdfs-gui: the status pages explain the refusal, and a dialog offers
to open the folder or choose another one, once per run of failures.
The folder picker refuses a folder with files in it.
Translated in all catalogs. Docs: BUGS (B277), CHANGELOG, FAQ,
TROUBLESHOOTING, CONFIGURATION.
