Ditch 5.6 user columns, if found

Summary:
Our admission control stuff barfs all over these if it finds them.
So, if found, nuke them.  We can't use them in 5.1 anyway.

This should allow downgrades from 5.6 to 5.1 after 5.6's mysql_upgrade,
including both in-place downgrades, and xtrabackup runs from 5.6 -> 5.1.

Test Plan:
Jenkins will test for mtr test failures when I post this diff.

Did this on test intance:
	Upgraded to 5.6 test rpms.
	Downgraded to 5.1 test rpms with this change.
	SHOW COLUMNS IN mysql.user;
		Saw these three (5.6 additions, trigger the problems):
			plugin
			authentication_string
			password_expired
	Ran mysql_upgrade (included in the 5.1 "-server" test rpm).
	SHOW COLUMNS IN mysql.user;
		Saw the three had been removed.  :)

Rudy, please really crush-test this with the new xtrabackup, test rpms:
	~steaphan/rpms/fb-mysql-*5.1.63-201304071118.dev.steaphan*.rpm

Reviewers: rudradevbasak, jtolmer

Reviewed By: jtolmer

client/mysql_upgrade.c

@@ -716,6 +716,7 @@ static const char *expected_errors[]=
   "ERROR 1060", /* Duplicate column name */
   "ERROR 1061", /* Duplicate key name */
   "ERROR 1054", /* Unknown column */
+  "ERROR 1091", /* Can't DROP 'column' */
   0
 };
 

scripts/mysql_fix_privilege_tables.sql

@@ -136,6 +136,9 @@ ALTER TABLE user add File_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL;
 SET @hadGrantPriv:=0;
 SELECT @hadGrantPriv:=1 FROM user WHERE Grant_priv LIKE '%';
 
+ALTER TABLE user DROP plugin;
+ALTER TABLE user DROP authentication_string;
+ALTER TABLE user DROP password_expired;
 ALTER TABLE user add Grant_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add References_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Index_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Alter_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL;
 ALTER TABLE host add Grant_priv enum('N','Y') NOT NULL,add References_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Index_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Alter_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL;
 ALTER TABLE db add Grant_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add References_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Index_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Alter_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL;

scripts/mysql_fix_privilege_tables_sql.c

@@ -157,6 +157,9 @@ const char* mysql_fix_privilege_tables={
 "SET @hadGrantPriv:=0;\n "
 "SELECT @hadGrantPriv:=1 FROM user WHERE Grant_priv LIKE '%';\n "
 "\n "
+"ALTER TABLE user DROP plugin;\n"
+"ALTER TABLE user DROP authentication_string;\n"
+"ALTER TABLE user DROP password_expired;\n"
 "ALTER TABLE user add Grant_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add References_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Index_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Alter_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL;\n "
 "ALTER TABLE host add Grant_priv enum('N','Y') NOT NULL,add References_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Index_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Alter_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL;\n "
 "ALTER TABLE db add Grant_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add References_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Index_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Alter_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL;\n "

scripts/mysql_system_tables_fix.sql

@@ -33,6 +33,9 @@ ALTER TABLE user add File_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL;
 SET @hadGrantPriv:=0;
 SELECT @hadGrantPriv:=1 FROM user WHERE Grant_priv LIKE '%';
 
+ALTER TABLE user DROP plugin;
+ALTER TABLE user DROP authentication_string;
+ALTER TABLE user DROP password_expired;
 ALTER TABLE user add Grant_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add References_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Index_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Alter_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL;
 ALTER TABLE host add Grant_priv enum('N','Y') NOT NULL,add References_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Index_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Alter_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL;
 ALTER TABLE db add Grant_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add References_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Index_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL,add Alter_priv enum('N','Y') COLLATE utf8_general_ci NOT NULL;