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

.vscode/settings.json

@@ -10,6 +10,7 @@
         "Mbps",
         "nolint",
         "Owncast",
+        "ppid",
         "preact",
         "RTMP",
         "rtmpserverport",
@@ -20,4 +21,4 @@
         "Warnf",
         "Warnln"
     ]
-}
+}