Fix reverse_tcp listener's handling of disconnects before staging

[bcook-r7]

If we get a tcp disconnect before we finish accepting the socket, the reverse_tcp handler thread breaks out of its internal loop, and we end up with a zombie handler that accepts at the TCP level, but does not actually work any more. Fix #6497

This PR changes the behavior to continue instead. While I was poking around, I also included a commit to tidy up some of the odd ruby style.

Verification

• Reverse_tcp payloads stage and continue working as expected
• start a reverse_tcp handler:

./msfconsole -qx 'use exploit/multi/handler; set payload windows/meterpreter/reverse_tcp; set lhost 192.168.56.1; run -j -z'

• scan the heck out of it

while true; nmap 192.168.56.1 -p 4444; done

• verify that you can stage a new payload after (or during) the above abuse

[OJ]

I'll take this one on today as I have a case where I can repro.

[OJ]

Confirmed breakage on master using @bcook-r7's testing technique as well as in $PRODUCTION. Testing fix now.

[OJ]

From my testing on my local machine, and in $PRODUCTION, this fix doesn't appear to resolve the issue. I'm still unable to connect (both after the intenstive scan or during). I have to restart the handler to get anything to function again.

[busterb]

Confirmed - it happened to survive on my Mac earlier (YMMV), but broke on Linux at home too.

[OJ]

[busterb]

You're not my dad!

Looks like we shoot ourselves twice - updated with a more robust (and hopefully less noisy on the logs as well) version.

[OJ]

Testing!

[OJ]

MSF be like...

You be like...

Everyone be like...

Much better. Awesome work mate.

[busterb]

[OJ]

Wow, you're quick @zeroSteiner, beat me to the label removal!

Thanks for the awesome work Brent. Looking good now.

[busterb]

For the label change, I think we have a web hook to remove those tags automatically; not sure why or how zeroSteiner's avatar shows up there, but I don't think it's correct. I put in a support query to find out.

It's part of a trial integration we're testing out to help manage and sort PRs and issues better: https://waffle.io/rapid7/metasploit-framework

[OJ]

Ah! Waffle.. yup. Nice.