Remove nfs dependency from org.jnode.fs.command plugin

Although mount does depend on nfs when mounting nfs volumes it seems silly that the command fs commands should rely on such a deep dependency tree that nfs has. With the current plugin, you cant have fs commands without including a large chunk of net plugins.

Moving mount to its own plugin doesn't really solve the issue either, because then mount is still going to depend on nfs, and you wont have the command without nfs. A better solution might be to remove the dependency from the plugin(what im currently doing), and have mount check if the nfs plugin is available before mounting an nfs volume. Then it will still be available to mount local volumes, and can simply produce an error if the nfs plugin is not available.