data dictionary confusion

On file systems with case insensitive file names, and
lower_case_table_names set to '2', the server could crash
due to a table definition cache inconsistency.  This is 
the default setting on MacOSX, but may also be set and
used on MS Windows.

The bug is caused by using two different strategies for
creating the hash key for the table definition cache, resulting
in failure to look up an entry which is present in the cache,
or failure to delete an existing entry.  One strategy was to
use the real table name (with case preserved), and the other
to use a normalized table name (i.e a lower case version).

This is manifested in two cases.  One is  during 'DROP DATABASE', 
where all known files are removed.  The removal from
the table definition cache is done via a generated list of
TABLE_LIST with keys (wrongly) created using the case preserved 
name.  The other is during CREATE TABLE, where the cache lookup
is also (wrongly) based on the case preserved name.
   
The fix was to use only the normalized table name when
creating hash keys.


sql/sql_db.cc:
  Normalize table name (i.e lower case it)
sql/sql_table.cc:
  table_name contains the normalized name
  alias contains the real table name