ECPG: Reduce the scope of va_start/va_end in ECPGfetch(), so
error paths need less code.
ECPG: Pass the SCROLL/NO SCROLL/unspecified to ECPGopen()
and pass the direction and the amount of tuples to ECPGfetch().
Use this information to reduce network roundtrip to the backend
in case a known-non-scrollable cursor and backward movement
are used together.