pageinspect: More type-sanity surgery on the new hash index code.

pageinspect: More type-sanity surgery on the new hash index code.

Uniformly expose unsigned quantities using the next-wider signed
integer type (since we have no unsigned types at the SQL level).
At the SQL level, this results a change to report itemoffset as
int4 rather than int2.  Also at the SQL level, report one value
that is an OID as type oid.  Under the hood, uniformly use macros
that match the SQL output type as to both width and signedness.