MDEV-29481 mariadb-upgrade prints confusing statement

This is a new version of the patch instead of the reverted:

  MDEV-28727 ALTER TABLE ALGORITHM=NOCOPY does not work after upgrade

Ignore the difference in key packing flags HA_BINARY_PACK_KEY and HA_PACK_KEY
during ALTER to allow ALGORITHM=INSTANT and ALGORITHM=NOCOPY in more cases.

If for some reasons (e.g. due to a bug fix such as MDEV-20704) these
cumulative (over all segments) flags in KEY::flags are different for
the old and new table inside compare_keys_but_name(), the difference
in HA_BINARY_PACK_KEY and HA_PACK_KEY in KEY::flags is not really important:

MyISAM and Aria can handle such cases well: per-segment flags are stored in
MYI and MAI files anyway and they are read during ha_myisam::open()
ha_maria::open() time. So indexes get opened with correct per-segment
flags that were calculated during the table CREATE time, no matter
what the old (CREATE time) and new (ALTER TIME) per-index compression
flags are, and no matter if they are equal or not.

All other engine ignore key compression flags, so this change
is safe for other engines as well.

mysql-test/main/alter_table_upgrade_myisam_debug.test

@@ -0,0 +1,19 @@
+--source include/have_debug.inc
+
+--echo #
+--echo # Start of 10.4 tests
+--echo #
+
+--echo #
+--echo # MDEV-29481 mariadb-upgrade prints confusing statement
+--echo #
+
+let $table= std_data/mysql_upgrade/mdev29481_100104_myisam;
+let $EXT_DAT= MYD;
+let $EXT_IDX= MYI;
+SET @debug_key_flags=TRUE;
+--source alter_table_upgrade_mdev29481_myisam_aria.inc
+
+--echo #
+--echo # End of 10.4 tests
+--echo #