MDEV-6068 Upgrade removes all changes to 'mysql' database

10.0 variation of the problem was that system tables were altered
during mysql_upgrade process using old (smaller) column lengths. 
At the end the tables were altered again, so the structure was restored,
but if there were long values before the upgrade, they were truncated.
Fixed by using correct column length in alter statements.

scripts/mysql_system_tables_fix.sql

@@ -62,9 +62,9 @@ ALTER TABLE tables_priv
 ALTER TABLE tables_priv
   MODIFY Host char(60) NOT NULL default '',
   MODIFY Db char(64) NOT NULL default '',
-  MODIFY User char(16) NOT NULL default '',
+  MODIFY User char(80) NOT NULL default '',
   MODIFY Table_name char(64) NOT NULL default '',
-  MODIFY Grantor char(77) NOT NULL default '',
+  MODIFY Grantor char(141) NOT NULL default '',
   ENGINE=MyISAM,
   CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin;
 
@@ -90,7 +90,7 @@ ALTER TABLE columns_priv
 ALTER TABLE columns_priv
   MODIFY Host char(60) NOT NULL default '',
   MODIFY Db char(64) NOT NULL default '',
-  MODIFY User char(16) NOT NULL default '',
+  MODIFY User char(80) NOT NULL default '',
   MODIFY Table_name char(64) NOT NULL default '',
   MODIFY Column_name char(64) NOT NULL default '',
   ENGINE=MyISAM,
@@ -161,7 +161,7 @@ alter table func comment='User defined functions';
 # and reset all char columns to correct width
 ALTER TABLE user
   MODIFY Host char(60) NOT NULL default '',
-  MODIFY User char(16) NOT NULL default '',
+  MODIFY User char(80) NOT NULL default '',
   ENGINE=MyISAM, CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin;
 ALTER TABLE user
   MODIFY Password char(41) character set latin1 collate latin1_bin NOT NULL default '',
@@ -191,7 +191,7 @@ ALTER TABLE user
 ALTER TABLE db
   MODIFY Host char(60) NOT NULL default '',
   MODIFY Db char(64) NOT NULL default '',
-  MODIFY User char(16) NOT NULL default '',
+  MODIFY User char(80) NOT NULL default '',
   ENGINE=MyISAM, CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin;
 ALTER TABLE db
   MODIFY  Select_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL,
@@ -449,7 +449,7 @@ ALTER TABLE proc CONVERT TO CHARACTER SET utf8;
 ALTER TABLE proc  MODIFY db
                          char(64) collate utf8_bin DEFAULT '' NOT NULL,
                   MODIFY definer
-                         char(77) collate utf8_bin DEFAULT '' NOT NULL,
+                         char(141) collate utf8_bin DEFAULT '' NOT NULL,
                   MODIFY comment
                          char(64) collate utf8_bin DEFAULT '' NOT NULL;