bin/plugin removes itself

If you call `bin/plugin --remove es-plugin` the plugin got removed but the file `bin/plugin` itself was also deleted.

We now don't allow the following plugin names:

* elasticsearch
* elasticsearch-service-x86.exe
* plugin
* elasticsearch-service-mgr.exe
* elasticsearch.bat
* plugin.bat
* elasticsearch-service-x64.exe
* elasticsearch.in.sh
* service.bat

Closes #6745
can we put them all in a `private static final Set<String> BLACKLIST` and then just do 
```
if (Strings.hasLength(name) && BLACKLIST.contains(name.toLowerCase(Locale.ROOT))) {
   throw new ElasticsearchIllegalArgumentException("Illegal plugin name: " + name);
}