Add stageless meterpreter payload support

[OJ]

For all the detail, please see the MSF side of this PR: https://github.com/rapid7/metasploit-framework/pull/4925

Given that the MSF side of this PR is required to do the testing that's where all the details are. Sorry for the scant detail here, but I felt that keeping it in the one spot made the most sense.

Note: there are a few formatting changes, style updates, documentation updates, etc in here. I realised later in the game that some of this might overlap with the changes that @bcook-r7 is making in his effort to replace openssl with libressl. Happy to help with the merging of that effort (sorry for any merge conflicts for you Brent).

Thanks!

[metasploit-public-bot]

Test PASSED. Refer to this link for build results (access rights to CI server needed): https://ci.metasploit.com//job/GPR-MeterpreterWin/153/ Test PASSED.

[bcook-r7]

No sweat! To the first merger lands the spoils!

Really cool work, by the way.

[bcook-r7]

By the way, I'm fixing the posix build failure here, doing a little rearranging to make it nice. I'll send a PR when its done.

[OJ]

The POSIX build failed? Crap! Thanks for the help Brent!

[busterb]

I went a little too far with refactoring though. Let me reel it in and send you what I had earlier that duplicates code a little, but gets us over this hump for this PR.

[busterb]

https://github.com/OJ/meterpreter/pull/4

[OJ]

Thanks @bcook-r7 ! I'll look at this first thing.

[metasploit-public-bot]

Test PASSED. Refer to this link for build results (access rights to CI server needed): https://ci.metasploit.com//job/GPR-MeterpreterWin/154/ Test PASSED.

[bcook-r7]

Looking good on this side, no regressions. I'll land so we can build a test gem.

[OJ]

Awesome, thanks @bcook-r7 !