Much of the test grid disabled: reinstate and rebase before merging.

Travis web interface doesn't allow anymore to set an empty string as
variable value.

Many tests were doing it manually, some weren't doing it and resulted in
failure if run in different order.

Close #948

Oid is defined as unsigned 32. On some Python implementations (probably
the ones where maxint = 2 ** 31) this can cause int overflow for large
values (see #961). On my 64 box it doesn't seem the case.

Oid handling was sloppy here and there (messages, casts...): trying to
use uint everywhere, and added a couple of helper macros to treat Oid
consistently.

Close #961.