Problem was that the mix of handlers was not consistent between
CREATE and ALTER

changed so that it works like:
    - All partitions must use the same engine
      AND it must be the same as the table.
    - if one does NOT specify an engine on the table level
      then one must either NOT specify any engine on any
      partition/subpartition OR for ALL partitions/subpartitions

Note: that after a table have been created, the storage engine
is specified for all parts of the table (table/partition/subpartition)
and so when using alter, one does not need to specify it (unless one
wants to change the storage engine, then one have to specify it on the
table level)


mysql-test/r/partition.result:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  test result updated
mysql-test/r/partition_innodb.result:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  test result updated
mysql-test/suite/ndb/r/ndb_partition_key.result:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  test result updated
mysql-test/suite/ndb/t/ndb_partition_key.test:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  test case update
mysql-test/suite/parts/inc/partition_engine.inc:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  test case updated
mysql-test/suite/parts/r/ndb_partition_key.result:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  test result updated
mysql-test/suite/parts/r/partition_engine_innodb.result:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  test result updated
mysql-test/suite/parts/r/partition_engine_myisam.result:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  test result updated
mysql-test/suite/parts/t/ndb_partition_key.test:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  test case updated
mysql-test/t/partition.test:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  test case updated
mysql-test/t/partition_innodb.test:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  test case updated
sql/partition_info.cc:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  moved the check_engine_condition here from sql_partition.cc
  created a new check_engine_mix from check_native_partitioned in
  sql_partition.cc
sql/partition_info.h:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  non static function check_engine_mix (now used in sql_partition.cc)
sql/sql_partition.cc:
  Bug#31931 Partitions: unjustified 'mix of handlers' error message
  moved check_engine_condition to partition_info.cc and moved out some
  common code in check_native_partitioned to check_engine_mix in
  partition_info.cc