mount: disable mount for freebsd

The upstream library rclone uses for rclone mount no longer supports
freebsd. Not only is it broken, but it no longer compiles.

This patch disables rclone mount for freebsd.

However all is not lost for freebsd users - compiling rclone with the
`cmount` tag, so `go install -tags cmount` will install a working
`rclone mount` command which uses cgofuse and the libfuse C library
directly.

Note that the binaries from rclone.org will not have mount support as
we don't have a freebsd build machine in CI and it is very hard to
cross compile cmount.

See: https://github.com/bazil/fuse/issues/280
Fixes #5843

cmd/mount/handle.go

@@ -1,5 +1,5 @@
-//go:build linux || freebsd
-// +build linux freebsd
+//go:build linux
+// +build linux
 
 package mount