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/cgroups-agent/cgroups-agent.c

@@ -52,7 +52,7 @@ int main(int argc, char *argv[]) {
 
         l = strlen(argv[1]);
 
-        n = sendto(fd, argv[1], l, 0, &sa.sa, offsetof(struct sockaddr_un, sun_path) + strlen(sa.un.sun_path));
+        n = sendto(fd, argv[1], l, 0, &sa.sa, SOCKADDR_UN_LEN(sa.un));
         if (n < 0) {
                 log_debug_errno(errno, "Failed to send cgroups agent message: %m");
                 return EXIT_FAILURE;