tree-wide: introduce new SOCKADDR_UN_LEN() macro, and use it everywhere

The macro determines the right length of a AF_UNIX "struct sockaddr_un" to pass to
connect() or bind(). It automatically figures out if the socket refers to an
abstract namespace socket, or a socket in the file system, and properly handles
the full length of the path field.

This macro is not only safer, but also simpler to use, than the usual
offsetof() + strlen() logic.

src/import/importd.c

@@ -677,7 +677,7 @@ static int manager_new(Manager **ret) {
         (void) mkdir_parents_label(sa.un.sun_path, 0755);
         (void) unlink(sa.un.sun_path);
 
-        if (bind(m->notify_fd, &sa.sa, offsetof(union sockaddr_union, un.sun_path) + strlen(sa.un.sun_path)) < 0)
+        if (bind(m->notify_fd, &sa.sa, SOCKADDR_UN_LEN(sa.un)) < 0)
                 return -errno;
 
         if (setsockopt(m->notify_fd, SOL_SOCKET, SO_PASSCRED, &one, sizeof(one)) < 0)