Ensure xmloption = content while restoring pg_dump output.

In combination with the previous commit, this ensures that valid XML
data can always be dumped and reloaded, whether it is "document"
or "content".

Discussion: https://postgr.es/m/CAN-V+g-6JqUQEQZ55Q3toXEN6d5Ez5uvzL4VR+8KtvJKj31taw@mail.gmail.com

src/bin/pg_dump/pg_backup_archiver.c

@@ -3179,6 +3179,9 @@ _doSetFixedOutputState(ArchiveHandle *AH)
 	/* Make sure function checking is disabled */
 	ahprintf(AH, "SET check_function_bodies = false;\n");
 
+	/* Ensure that all valid XML data will be accepted */
+	ahprintf(AH, "SET xmloption = content;\n");
+
 	/* Avoid annoying notices etc */
 	ahprintf(AH, "SET client_min_messages = warning;\n");
 	if (!AH->public.std_strings)