cleanup patches for dshash

While working on the dynamic shared memory registry, I noticed a couple of
potential improvements for code that uses dshash tables.

* A couple of dshash_create() callers pass in 0 for the "void *arg"
  parameter, which seemed weird.  I incorrectly assumed this was some sort
  of flags parameter until I actually looked at the function signature.
  IMHO we should specify NULL here if arg is not used.  0001 makes this
  change.  This is admittedly a nitpick.

* There are no dshash compare/hash functions for string keys.  0002
  introduces some that simply forward to strcmp()/string_hash(), and it
  uses them for the DSM registry's dshash table.  This allows us to remove
  the hacky key padding code for lookups on this table.

Thoughts?

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com