Move migration stub generation into MSF

[OJ]

This code adds support for transport-specific migration stubs to be generated in MSF rather than having them hard-coded in Meterpreter. The accompanying PR for Meterpreter is here: rapid7/metasploit-payloads#155

This is a subset of work done in #6860, and the aim is to reduce that PR into more reasonable chunks so that it has a hope of landing.

Changes include:

• Creation of new migration payloads that are built on the fly when migration is happening.
• Payloads are arch and transport specific (required for things like named pipes down the track as each transport has a nuance to its migration).
• Some other code tidying/refactoring, but not much.

Verification

• Generate http/s and tcp payloads for Windows x86 and Windows x64
• Set up respective handlers.
• Migrate from x86 to x86. It should work.
• Migrate from x86 to x64. It should work.
• Migrate from x64 to x64. It should work.
• Migrate from x64 to x86. It should work.

Done!

Ping @sempervictus who was particularly interested in this.

[OJ]

Note: I'll need to update this PR with the Meterp gems once the payloads have been built, and also update the payload sizes. This build might not go green to begin with.

[busterb]

Thanks. Migration should get even more interesting in the future:
https://www.youtube.com/watch?v=A_GlGglbu1U

[wwebb-r7]

Release Notes

The migration stub code generation from Meterpreter has been relocated into Metasploit Framework.

[OJ]

Thanks!

[bennofs]

did this remove support for linux migration?

[OJ]

Linux migration hasn't worked for quite a long time. This PR was no the cause. I'm not sure what the status is with regards to reimplementing it. @busterb any thoughts?