Reject zero or negative BY step in plpgsql integer FOR-loops, and behave

Reject zero or negative BY step in plpgsql integer FOR-loops, and behave
sanely if the loop value overflows int32 on the way to the end value.
Avoid useless computation of "SELECT 1" when BY is omitted.  Avoid some
type-punning between Datum and int4 that dates from the original coding.