-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

main.c

@@ -547,7 +547,7 @@ void run_key_handler(const char *key, unsigned int mask)
 	for (f = i = 0; f < fcnt; i++) {
 		if ((marked && (files[i].flags & FF_MARK)) || (!marked && i == fileidx)) {
 			stat(files[i].path, &oldst[f]);
-			fprintf(pfs, "%s\n", files[i].name);
+			fprintf(pfs, "%s%c", files[i].name, options->using_null ? '\0' : '\n');
 			f++;
 		}
 	}