Re: add function for creating/attaching hash table in DSM registry

Here is a new patch with GetNamedDSA() added.  A couple notes:

* I originally wanted to use GetNamedDSA() within GetNamedDSMHash(), but
  that would probably lead to two registry entries per dshash table, and it
  didn't really save all that much code, anyway.  So, I didn't do that.

* Using a DSA from the registry is cumbersome.  You essentially need
  another batch of shared memory to keep track of the pointers and do
  locking, so it might not be tremendously useful on its own.  AFAICT the
  easiest thing to do is to store the DSA pointers in a dshash table, which
  is what I've done in the test.

-- 
nathan