-0 sends NULL separated file-list to key-handler

with this change `-0` is turned into a more generic switch which can be
used to send NULL-separated file-list to the key-handler as well.
this also means `-0` no longer implicitly enables `-o`

Closes: https://github.com/nsxiv/nsxiv/issues/140

nsxiv.h

@@ -256,7 +256,7 @@ struct opt {
 	char **filenames;
 	bool from_stdin;
 	bool to_stdout;
-	char stdout_separator;
+	bool using_null;
 	bool recursive;
 	int filecnt;
 	int startnum;