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
Good point. To ensure that `null` and blanks are captured, then we can do

```java
if ( ! Strings.hasText(name) || BLACKLIST.contains(name.toLowerCase(Locale.ROOT))) {
   throw new ...
}
```