syslog input with controll characters

## Context
It might happen that someone sends syslog messages with controll characters to Graylog (see https://community.graylog.org/t/multiline-fragmented-rsyslog-events/10159 ) we need to decide if Graylog should accept and work with them.

If the decision is made to ignore, than we need to document that clearly.

## Your Environment

* Graylog Version: 3.0