make 'PrivateRefCount' 32 bits wide

This patch changes PrivateRefCount and LocalRefCount in the bufmgr from
being arrays of long to arrays of int32, per earlier discussion. This
should save a little memory on 64LP machines, and there's no way we need
to track > 2^31 references to a single buffer.

I also fixed the rather widespread assumption that a BufferDesc's
refcount is signed; it is not (and hence the correct printf formatting
code is %u, not %d).

I also changed ShowPinTrace to a bool, and removed some multi-line
string literals (yuck!)

Barring any objections, I intend to apply this patch within 24 hours.

-Neil