multiple columns in the partition key

ndb crash if duplicate columns in the partitioning key.

Backport from mysql-5.1-telco-7.0, see bug#53354.

Changed from case sensitive field name comparision
to non case sensitive too.

mysql-test/r/partition_error.result:
  updated result
mysql-test/t/partition_error.test:
  Added test for the error in non-ndb partitioned table.
sql/sql_partition.cc:
  Added check for duplicated field names in the
  partitioning key.