trx_rseg_header_create(): Add a parameter for the value that is
to be written to TRX_RSEG_MAX_TRX_ID. If we omit this write, then
the updated test innodb.undo_truncate will fail for the 4k, 8k, 16k
page sizes. This was broken ever since
commit 947efe17 (MDEV-15158)
removed the writes of transaction identifiers to the TRX_SYS page.

srv_do_purge(): Truncate undo tablespaces also during slow shutdown
(innodb_fast_shutdown=0).

Thanks to Krunal Bauskar for noticing this problem.