Metadata files have the file handle of their source file with
0x00000001 suffixed in big endian so we can look them up directly from
their file handles.
`--nfs-cache-type symlink` is similar to `--nfs-cache-type disk` in
that it uses an on disk cache, but the cache entries are held as
symlinks. Rclone will use the handle of the underlying file as the NFS
handle which improves performance.
This problem was caused by the defaults not being set for the options
after the conversion to the new config system in
28ba4b832d serve nfs: convert options to new style
This makes the nfs serve options globally available so nfsmount can
use them directly.
Fixes#8029
Summary:
In cases where cmount is not available in macOS, we alias nfsmount to mount command and transparently start the NFS server and mount it to the target dir.
The NFS server is started on localhost on a random port so it is reasonably secure.
Test Plan:
```
go run rclone.go mount --http-url https://beta.rclone.org :http: nfs-test
```
Added mount tests:
```
go test ./cmd/nfsmount
```
