Auto updater APIs (#1523)

* APIs for querying and executing an update in place.

For #924

* Use the process pid to query systemd for status

* Use parent pid and invocation ID to guess if running from systemd

* Stream cmd output to client + report errors

* Update comment to refer to INVOCATION_ID

config/updaterConfig_enabled.go

@@ -0,0 +1,8 @@
+//go:build enable_updates
+// +build enable_updates
+
+package config
+
+func init() {
+	EnableAutoUpdate = true
+}