Postgres provider to use gda-srv lib

[Vivien Malerba <malerba linuxave net>]

Hi Rodrigo!

I've started to modify the Postgres provider to use the gda-server lib. I used
the mysql provider on the way to compare. I found some points where the two
providers do not behave the same way, and points where we should define some
more things. Here are my thoughts:

1) in the function gda_mysql_connection_free(), maybe you it should be added:
 gda_server_connection_set_user_data(cnc, NULL) so we don't have a wild pointer.

2) in the functions gda_*_connection_get_gda_type(): I propose that the
returned value be GDA_TypeNull if there is an error (cnc NULL or user data
NULL), and GDA_TypeVarchar if the type is unknown but there is no error.

3) the returned value from the function gda_mysql_connection_get_c_type() is
not well defined. So I propose that we define them as I did with Postgres 
(Gda_POSTGRES_CType enumeration) once for everyone. I don't know exactly where
to put this.

At the moment I have something quite big which does not even compile!
Tell me what you think of all this.

Cheers,

Vivien