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/core/execute.c

@@ -271,7 +271,7 @@ static int connect_journal_socket(int fd, uid_t uid, gid_t gid) {
                 }
         }
 
-        r = connect(fd, &sa.sa, offsetof(struct sockaddr_un, sun_path) + strlen(sa.un.sun_path));
+        r = connect(fd, &sa.sa, SOCKADDR_UN_LEN(sa.un));
         if (r < 0)
                 r = -errno;