Extract process-management functionality into a new lib/process

Note that procmon does *not* go here, since procmon needs to
integrate with the event loop.

src/lib/process/daemon.h

@@ -0,0 +1,12 @@
+/* Copyright (c) 2003-2004, Roger Dingledine
+ * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
+ * Copyright (c) 2007-2018, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#ifndef TOR_DAEMON_H
+#define TOR_DAEMON_H
+
+void start_daemon(void);
+void finish_daemon(const char *desired_cwd);
+
+#endif