whenever possible, partitioning should use the full
partition plugin name, not the one byte legacy code.

Normally, ha_partition can get the engine plugin from
table_share->default_part_plugin.

But in some cases, e.g. in DROP TABLE, the table isn't
opened, table_share is NULL, and ha_partition has to parse
the frm, much like dd_frm_type() does.

temporary_tables.cc, sql_table.cc:

When dropping a table, it must be deleted in the engine
first, then frm file. Because frm can be the only true
source of metadata that the engine might need for DROP.

table.cc:

when opening a partitioned table, if the engine for
partitions is not found, do not fallback to MyISAM.