Patch to define DB_TYPE_TOKUDB

Let TokuDB have a well known plugin number rather than a dynamically assigned plugin number. As an alternative, TokuDB can be compiled with DB_TYPE_UNKNOWN which requests a dynamic plugin number.

MySQL has problems with drop table which would delete the frm but not the underlying TokuDB files. The drop table would use the plugin number from the frm to find the storage engine. If the storage engine's plugin number changed since the frm was created, then drop table would screw up. MariaDB claims to have fixed this problem.

1. create a tokudb table 'test.t'.
2. shutdown mysqld
3. set its frm byte[3] = 45, as if the tokudb plugin number was originally 45. see attached program.
4. start mysql
5. drop table 'test.t'.  returns ok, frm is gone, tokudb files are still there.