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/fsck/fsck.c

@@ -262,7 +262,7 @@ static int fsck_progress_socket(void) {
         if (fd < 0)
                 return log_warning_errno(errno, "socket(): %m");
 
-        if (connect(fd, &sa.sa, offsetof(struct sockaddr_un, sun_path) + strlen(sa.un.sun_path)) < 0) {
+        if (connect(fd, &sa.sa, SOCKADDR_UN_LEN(sa.un)) < 0) {
                 r = log_full_errno(errno == ECONNREFUSED || errno == ENOENT ? LOG_DEBUG : LOG_WARNING,
                                    errno, "Failed to connect to progress socket %s, ignoring: %m", sa.un.sun_path);
                 safe_close(fd);