Move confline.c to lib/encoding: it is about encoding key-value pairs

Also, move "unescape_string()" to encoding too, since it's about
encoding data as C strings.

src/lib/encoding/cstring.h

@@ -0,0 +1,13 @@
+/* Copyright (c) 2001 Matej Pfajfar.
+ * Copyright (c) 2001-2004, 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_CSTRING_H
+#define TOR_CSTRING_H
+
+#include <stddef.h>
+const char *unescape_string(const char *s, char **result, size_t *size_out);
+
+#endif /* !defined(TOR_CSTRING_H) */