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
If we did strncmp(argv[0], kNsEnter, strlen(kNsEnter)) != 0 then we could support registering for eg nsenter-exec, nsenter-mknod init functions.  Just a thought..