work in 5.1.40)

MERGE engine fails to open child table from a different
database if child table/database name contains characters
that are subject for table name to filename encoding
(WL1324).

Another problem is that MERGE engine didn't properly open
child table from the same database if child table name
contains characters like '/', '#'.

The problem was that table name to file name encoding was
applied inconsistently:
* On CREATE: encode table name + database name if child
  table is in different database; do not encode table
  name if child table is in the same database;
* No decoding on open.

With this fix child table/database names are always
encoded on CREATE and decoded on open. Compatibility
with older tables preserved.

Along with this patch comes fix for SHOW CREATE TABLE,
which used to show child table/database path instead
of child table/database names.