table with backticks

(Thanks to Lu Jingdong, though I did not take his patch directly, as
it contained a significant flaw.)

It wasn't a backtick/parsing problem.  We merely didn't anticipate
and allocate enough space to handle the optional "#mysql50#" table-
name prefix. 

Now, allocate that extra space in case we need it when we look up 
a legacy table to get its file's name.


mysql-test/r/drop.result:
  Verify that databases with old-style files can be removed.
mysql-test/t/drop.test:
  Verify that databases with old-style files can be removed.
sql/sql_db.cc:
  Extend the size of the memory that holds the table's name, so that
  the legacy "mysql50" prefix fits.