a couple of small cleanup patches for DSM-related code

0001 changes test_dsm_registry.c to use PG_GETARG_INT32 and
PG_RETURN_INT32.  The installation script and the C code both used signed
integers, so I'm not sure why I used PG_GETARG/RETURN_UINT32 in commit
8b2bcf3.  I'm planning to back-patch this one to v17, where the DSM
registry was first introduced.

0002 follows commit 5fe08c0's example and changes some calls to
dshash_attach() and dsa_create_in_place() to use NULL instead of 0 for
pointer arguments.  I don't see any need to back-patch this one, but I also
don't see any need to wait for v19devel to commit it.

Any objections?

-- 
nathan