Move migration stub code to MSF

[OJ]

This commit changes the code so that the migration stubs are generated in MSF and are transport specific (so that we only do the work we need to).

This is a subset of the work that was written into the Named Pipe PR #101. I have pulled it into this PR so that:

1. It has a chance of getting landed.
2. It makes the other PR a little more tangible.
3. It'll allow me to take the next bit of Add named pipe channels, named pipe transport, and a small migration refactor #101 out as another piecemeal chunk.

This will also mean that:

• Meterpreter is slightly less fingerprintable.
• Slightly smaller too.
• MSF can generate migration stubs on the fly and perhaps vary them down the track.

For more details, please see the MSF side PR at this location: rapid7/metasploit-framework#7674

[busterb]

Rocking, thanks so much for making this easier to digest!

[OJ]

Thank you!