Cleanup: refactor shutdown and signal handling facility

This disentangles the following functions, which were previously all mixed together:

* 1) Waiting for jobs to terminate when shutting down
* 2) Handling signals in the Docker daemon
* 3) Per-subsystem cleanup handlers
* 4) pidfile management

Responsibilities are dispatched as follows:

* Signal traps are set in `main`, and trigger `engine.Shutdown`
* `engine.Shutdown` coordinates cleanup by waiting for jobs to complete, and calling shutdown handlers
* To perform cleanup at shutdown, each subsystem registers handlers with `engine.OnShutdown`
* `daemon` is one subsystem, so it registers cleanup via `engine.OnShutdown`.
* `daemon` owns the pidfile, which is used to lock access to `/var/lib/docker`. Part of its cleanup is to remove the pidfile.

Signed-off-by: Solomon Hykes <solomon@docker.com>
Remove unnecessary job "initserverpidfile"

That job was a hacky solution to a real race condition. This removes the
hack without re-introducing the race.

Signed-off-by: Solomon Hykes <solomon@docker.com>
Replace Server.IsRunning with Engine.IsShutdown

Signed-off-by: Solomon Hykes <solomon@docker.com>
Extra testing for engine.Shutdown

Signed-off-by: Solomon Hykes <solomon@docker.com>
Remove last trace of Daemon->Server dependency

This removes a shim `daemon.Server` interface which was used to start
separating Daemon from Server *gradually*, without getting cyclic
dependency errors.

Now that the last Daemon->Server dependency has been removed, we can
finally remove the shim. Yay!

Signed-off-by: Solomon Hykes <solomon@docker.com>
Remove unused field from engine.Job

Signed-off-by: Solomon Hykes <solomon@docker.com>
Jobs can be stopped (cooperatively) with job.Stop() and job.OnStop()

Signed-off-by: Solomon Hykes <solomon@docker.com>