Replace custom child reap code with go-reap

vendor/github.com/hashicorp/go-reap/README.md

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+# go-reap
+
+Provides a super simple set of functions for reaping child processes. This is
+useful for running applications as PID 1 in a Docker container.
+
+Note that a mutex is supplied to allow your application to prevent reaping of
+child processes during certain periods. You need to use care in order to
+prevent the reaper from stealing your return values from uses of packages like
+Go's exec. We use an `RWMutex` so that we don't serialize all of your
+application's execution of sub processes with each other, but we do serialize
+them with reaping. Your application should get a read lock when it wants to do
+a wait and be safe from the reaper.
+
+This should be supported on most UNIX flavors, but is not supported on Windows
+or Solaris. Unsupported platforms have a stub implementation that's safe to call,
+as well as an API to check if reaping is supported so that you can produce an
+error in your application code.
+
+Documentation
+=============
+
+The full documentation is available on [Godoc](http://godoc.org/github.com/hashicorp/go-reap).
+
+Example
+=======
+
+Below is a simple example of usage
+
+```go
+// Reap children with no control or feedback.
+go ReapChildren(nil, nil, nil)
+
+// Get feedback on reaped children and errors.
+if reap.IsSupported() {
+	pids := make(reap.PidCh, 1)
+	errors := make(reap.ErrorCh, 1)
+	done := make(chan struct{})
+	var reapLock sync.RWMutex
+	go ReapChildren(pids, errors, done, &reapLock)
+	// ...
+	close(done)
+} else {
+	fmt.Println("Sorry, go-reap isn't supported on your platform.")
+}
+```
+