Fix pageinspect's heap_page_item to return infomasks as 32 bit values

HeapTupleHeader's t_infomask and t_infomask2 are defined as 16-bit
unsigned integers, so when the 16th bit was set, heap_page_item was
returning them as negative values, which was ugly.

The change to pageinspect--unpackaged--1.0.sql allows a module upgraded
from 9.0 to be cleanly updated from the previous definition.