Add some pieces of an IPC framework that will eventually allow ebuild

Add some pieces of an IPC framework that will eventually allow ebuild
processes can to communicate with portage's main python process.

Here are a few possible uses:

1) Robust subshell/subprocess die support. This allows the ebuild
   environment to reliably die without having to rely on signal IPC.

2) Delegation of portageq calls to the main python process, eliminating
   performance and userpriv permission issues.

3) Reliable ebuild termination in cases when the ebuild has accidentally
   left orphan processes running in the backgraound (as in bug 278895).