Add a simpler constructor for hash tables which specifically does not
require specifying the initial hash size and uses simpler freeing
function.

The initial hash table size usually is not important as the hash table
is growing when it reaches certain number of entries in one bucket.
Additionally many callers pass in a random small number for ad-hoc table
use so using a central one will simplify things.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
ACKed-by: NEric Blake <eblake@redhat.com>