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
Suggest making this an ordered array/loop in the interest of maintainability:

```java
boolean forbidden = Strings.isNullOrEmpty(name);

String[] forbiddenNames = {
    "elasticsearch",
    "elasticsearch.bat",
    "elasticsearch.in.sh",
    "elasticsearch-service-x86.exe",
    "elasticsearch-service-x64.exe",
    "elasticsearch-service-mgr.exe",
    "plugin",
    "plugin.bat",
    "service.bat"
};

for (String forbiddenName : forbiddenNames) {
    if (forbiddenName.equalsIgnoreCase(name)) {
        forbidden = true;
        break;
    }
}

if (forbidden) {
    throw new ElasticsearchIllegalArgumentException("This plugin name is not allowed");
}
```