1.0.26.20: tighter VECTOR-PUSH-EXTEND argument type

1.0.26.20: tighter VECTOR-PUSH-EXTEND argument type
	The optional extension parameter must be a positive integer.
	... declare this in fndb;
	... fix the erroneous use in constraints (not only ensuring
		positivity, but also it's an extension not a new-length
		parameter).
	Issue brought to light by Peter Graves' XCL.