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.

mysql-test/r/merge.result:
  A test case for BUG#48265.
mysql-test/std_data/bug48265.frm:
  MERGE table from 5.0 to test fix for BUG#48265 compatibility.
mysql-test/t/merge.test:
  A test case for BUG#48265.
storage/myisammrg/ha_myisammrg.cc:
  On CREATE always write child table/database name encoded
  by table name to filename encoding to dot-MRG file.
  
  On open decode child table/database name.
  Compatibilty with previous versions preserved.
  
  Fixed ::append_create_info() to return child
  table/database name instead of path.
storage/myisammrg/myrg_open.c:
  Move if (has_path) branch from myrg_parent_open() to
  myisammrg_parent_open_callback. The callback function
  needs to know if child table was written along with
  database name to dot-MRG file. Needed for compatibility
  reasons.