add function for creating/attaching hash table in DSM registry

Libraries commonly use shared memory to store hash tables.  While it's
possible to set up a dshash table using the DSM registry today, doing so is
complicated; you need to set up two LWLock tranches, a DSA, and finally the
dshash table.  The attached patch adds a new function called
GetNamedDSMHash() that makes creating/attaching a hash table in the DSM
registry much easier.

-- 
nathan