basic/ordered-set: export networkd function to print string sets

Tests are added.

src/basic/ordered-set.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
+#include "fileio.h"
 #include "ordered-set.h"
 #include "strv.h"
 
@@ -63,3 +64,19 @@ int ordered_set_put_string_set(OrderedSet *s, OrderedSet *l) {
 
         return n;
 }
+
+void ordered_set_print(FILE *f, const char *field, OrderedSet *s) {
+        bool space = false;
+        Iterator i;
+        char *p;
+
+        if (ordered_set_isempty(s))
+                return;
+
+        fputs(field, f);
+
+        ORDERED_SET_FOREACH(p, s, i)
+                fputs_with_space(f, p, NULL, &space);
+
+        fputc('\n', f);
+}

src/basic/ordered-set.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
+#include <stdio.h>
+
 #include "hashmap.h"
 
 typedef struct OrderedSet OrderedSet;
@@ -56,6 +58,7 @@ int ordered_set_consume(OrderedSet *s, void *p);
 int ordered_set_put_strdup(OrderedSet *s, const char *p);
 int ordered_set_put_strdupv(OrderedSet *s, char **l);
 int ordered_set_put_string_set(OrderedSet *s, OrderedSet *l);
+void ordered_set_print(FILE *f, const char *field, OrderedSet *s);
 
 #define ORDERED_SET_FOREACH(e, s, i)                                    \
         for ((i) = ITERATOR_FIRST; ordered_set_iterate((s), &(i), (void**)&(e)); )