util: add minimal OrderedSet type

OrderedSet implements a Set-like structure, but maintains insertion
ordered. It is hence to Set what OrderedHashmap is for Hashmap.

Internally, this is only a wrapper around OrderedHashmap for now, but
this could one day be improved and be added to hashmap.c natively.

Makefile.am

@@ -792,6 +792,7 @@ libsystemd_shared_la_SOURCES = \
 	src/shared/siphash24.c \
 	src/shared/siphash24.h \
 	src/shared/set.h \
+	src/shared/ordered-set.h \
 	src/shared/fdset.c \
 	src/shared/fdset.h \
 	src/shared/prioq.c \

src/shared/ordered-set.h

@@ -0,0 +1,59 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+#pragma once
+
+/***
+  This file is part of systemd.
+
+  Copyright 2015 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include "hashmap.h"
+
+typedef struct OrderedSet OrderedSet;
+
+static inline OrderedSet* ordered_set_new(const struct hash_ops *ops) {
+        return (OrderedSet*) ordered_hashmap_new(ops);
+}
+
+static inline OrderedSet* ordered_set_free(OrderedSet *s) {
+        ordered_hashmap_free((OrderedHashmap*) s);
+        return NULL;
+}
+
+static inline OrderedSet* ordered_set_free_free(OrderedSet *s) {
+        ordered_hashmap_free_free((OrderedHashmap*) s);
+        return NULL;
+}
+
+static inline int ordered_set_put(OrderedSet *s, void *p) {
+        return ordered_hashmap_put((OrderedHashmap*) s, p, p);
+}
+
+static inline bool ordered_set_isempty(OrderedSet *s) {
+        return ordered_hashmap_isempty((OrderedHashmap*) s);
+}
+
+static inline void *ordered_set_iterate(OrderedSet *s, Iterator *i) {
+        return ordered_hashmap_iterate((OrderedHashmap*) s, i, NULL);
+}
+
+#define ORDERED_SET_FOREACH(e, s, i)                                    \
+        for ((i) = ITERATOR_FIRST, (e) = ordered_set_iterate((s), &(i)); (e); (e) = ordered_set_iterate((s), &(i)))
+
+DEFINE_TRIVIAL_CLEANUP_FUNC(OrderedSet*, ordered_set_free);
+
+#define _cleanup_ordered_set_free_ _cleanup_(ordered_set_freep)