fix(fuse): keep files offline in on-demand synced folders

`pdfs pin` and `pdfs unpin` resolved every path against the primary mount
with `rel_to_mount`. An absolute path in an on-demand synced folder such as
~/Pictures was refused as "not under the mountpoint", and a relative path
named a file in My files instead. A synced folder's file is a different
Drive node from a same-named file in My files, so keeping one offline did
nothing for the other, and every program reading the synced copy
downloaded it.

Pin and unpin now route through `route_to_mount`, like ls, refresh, rm and
mv since B86, and run on the mount that owns the path. A path in a mirror
folder gets an error saying its files are already on this computer. A pin
made through an on-demand folder records the absolute local path, so
`pdfs pins`, the prompt and the HUD open the file where it was kept; pins
under the primary mount still record the mount-relative path.

Fixes B281.

docs/CLI.md

@@ -102,7 +102,7 @@ commands are for scripts and for cases where the mount is not available.
 
 | Command | Description |
 |---|---|
-| `pdfs pin PATH` | Download a file completely and keep it on this computer. For a folder, its files download in the background (`pdfs transfers` shows the progress). |
+| `pdfs pin PATH` | Download a file completely and keep it on this computer. For a folder, its files download in the background (`pdfs transfers` shows the progress). PATH can be under the Proton Drive mount or in an on-demand synced folder. A file in a mirror folder is already on this computer. |
 | `pdfs unpin PATH` | Release it back to online only. A file inside a folder kept offline cannot be released on its own. |
 | `pdfs pins` | List files and folders kept available offline. |