1. BUG#21704 - Renaming column does not update FK definition

2. Changes in mysql-test/include/mtr_warnings.sql so that the testcase
   for BUG#21704 doesn't fail because of the warnings generated.

Detailed revision comments:

r5488 | vasil | 2009-07-09 19:16:44 +0300 (Thu, 09 Jul 2009) | 13 lines
branches/5.1:

Fix Bug#21704 Renaming column does not update FK definition

by checking whether a column that participates in a FK definition is being
renamed and denying the ALTER in this case.

The patch was originally developed by Davi Arnaut <Davi.Arnaut@Sun.COM>:
http://lists.mysql.com/commits/77714
and was later adjusted to conform to InnoDB coding style by me (Vasil),
I also added some more comments and moved the bug specific mysql-test to
a separate file to make it more manageable and flexible.