DIR-645 authentication.cgi bof - ROP to system - starting telnetd

This is the replacement of https://github.com/rapid7/metasploit-framework/pull/3368
I think a better way of proceeding would be:

* Use current ARCH_CMD payloads.
* If any of them work in your busybox environment:

a) Maybe create a new ARCH_CMD payload for the telnetd command? Looks like it works with the Msf::Handler::BindTcp handler, so could have sense. 
b) If you are allowed to run ```system```, maybe any of the current cmdstagers could be used to execute a native payload.

I would love to listen opinions from others here. It's just my opinion. The current method is error prone from the user point of view, which can select wrong payloads.