Refactor execin send config over pipe

This refactors the entire execin.go file.  You cannot see it very well in the diff but if you look at the file alone it's much better over what we had before. 

I also made the change to send the container configuration to the nsenter process over a syncpipe.

Fixes #65
Closes #129

ping @vishh
in the nsenter could you could look at the comment to it to determine an action to take.  I also this this suggestion.  I think `nsenter-mknod` would be very clean.  I'll make the change