Bug#4521: unique key prefix interacts poorly with utf8.

Fix for binary collations for MyISAM and HEAP BTREE.
This patch also changes trailing spaces behaviour for
binary collations. Binary collations now have PAD 
characteristic too.

mysql-test/r/endspace.result

@@ -19,7 +19,7 @@ select 'a  a' > 'a', 'a  \0' < 'a';
 1	1
 select binary 'a  a' > 'a', binary 'a  \0' > 'a', binary 'a\0' > 'a';
 binary 'a  a' > 'a'	binary 'a  \0' > 'a'	binary 'a\0' > 'a'
-1	1	1
+1	0	0
 create table t1 (text1 varchar(32) not NULL, KEY key1 (text1));
 insert into t1 values ('teststring'), ('nothing'), ('teststring\t');
 check table t1;