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
```
