fs-util: make symlink_idempotent() optionally create relative link

src/basic/fs-util.c

@@ -346,12 +346,27 @@ int touch(const char *path) {
         return touch_file(path, false, USEC_INFINITY, UID_INVALID, GID_INVALID, MODE_INVALID);
 }
 
-int symlink_idempotent(const char *from, const char *to) {
+int symlink_idempotent(const char *from, const char *to, bool make_relative) {
+        _cleanup_free_ char *relpath = NULL;
         int r;
 
         assert(from);
         assert(to);
 
+        if (make_relative) {
+                _cleanup_free_ char *parent = NULL;
+
+                parent = dirname_malloc(to);
+                if (!parent)
+                        return -ENOMEM;
+
+                r = path_make_relative(parent, from, &relpath);
+                if (r < 0)
+                        return r;
+
+                from = relpath;
+        }
+
         if (symlink(from, to) < 0) {
                 _cleanup_free_ char *p = NULL;