updated gdbm library

[Luiz Henrique de Figueiredo <lhf@...>]

I've updated my gdbm library. It's available as usual at
        http://www.tecgraf.puc-rio.br/~lhf/ftp/lua/

The main changes are:
 - metatable stuff moved to C, so it's independent of gdbm.lua (which is still
   needed for the proxy and for stuff).
 - added gdbm.fdesc.
 - added handler for fatal gdbm errors.

Enjoy.
--lhf