added option to exit with an warning, if there is output on STDERR (#1297)

We need the option  to bail out with an error, if there is something on
stdout.
E.g. someone is defining: 
``` 
stty erase ^H
```

without any checks in the users profile (.bashrc or .profile)
You can merge this Pull Request by running:

  git pull https://github.com/nafets/monitoring-plugins master

Or you can view, comment on it, or merge it online at:

  https://github.com/monitoring-plugins/monitoring-plugins/pull/1297

-- Commit Summary --

  * added option to exit with an warning, if there is output on STDERR

-- File Changes --

    M plugins/check_by_ssh.c (14)

-- Patch Links --

https://github.com/monitoring-plugins/monitoring-plugins/pull/1297.patch
https://github.com/monitoring-plugins/monitoring-plugins/pull/1297.diff

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https://github.com/monitoring-plugins/monitoring-plugins/pull/1297