mountpoint-util: rename fd_is_mount_point() to is_mount_point_at()

fd_* functions in our codebase usually mean fd-specific operations,
while this one actually takes openat()-style params. Rename it
accordingly hence.

src/shared/copy.c

@@ -1147,7 +1147,7 @@ static int fd_copy_directory(
                                 if (buf.st_dev != original_device)
                                         continue;
 
-                                r = fd_is_mount_point(dirfd(d), de->d_name, 0);
+                                r = is_mount_point_at(dirfd(d), de->d_name, 0);
                                 if (r < 0)
                                         return r;
                                 if (r > 0)