Add beginnings of an IPC helper library

This will use a native transport mechanism in order to avoid issues with
firewalls and sandboxes that trip on TCP (even on loopback).
Port agent to use Pipe as its transport
Port the Darwin backend to use the new Pipe transport
Improve selection of temporary directory for the agent on Darwin

When running as root it is preferable to use /private/var/root as that
directory is more likely to be allowed by sandboxed daemons.
Update gitignore
Improve the Pipe mach backend and allow instantiating the pipe transport
Fix the agent test regressions
Fix error reporting in the fruitjector
Make cancellable optional in the Pipe mach backend
Revert the Fruitjector API changes
Revert a left-over change
Add Windows build system for zed-pipe
Add boilerplate for a pipe backend for Windows
Fix winjector tests on 32-bit systems
Update WindowsHostSession to the new BaseDBusHostSession API
Complete most of the pipe backend for Windows
Fix Pipe mach backend regressions
Fix pipe permission issues on Windows by making the pipe world writable
Add an interactive test app for zed-pipe
Replace winipc with zed-pipe/GDBus and refactor a bit