Fix issues with using int for number of steps

Fix issues with using int for number of steps

Mostly we use a 64-bit integer, but we messed up a few
things.

During mdrun -rerun, edr writing complained about the negative step
number, implied it might be working around it, and threatened to
crash, which it can't do. Silenced the complaint during writing,
and reduced the scope of the message when reading.

Fixed TNG wrapper routines to pass a 64-bit integer like they should.

Made various infrastructure use gmx_int64_t for consistency, and noted
where in a few places the practical range of the value stored in such
a type is likely to be smaller. We can't extend the definition of XTC
or TRR, so we're stuck. TNG is already good, though.

Fixes #2006

Change-Id: If485d9d92cb4b99a3bbe25e8e9fa082fc3fccd5f