hash: extend module to deal with non-pointer keys

hash: extend module to deal with non-pointer keys

* lib/hash.c (hash_insert0): New interface, much like hash_insert
but that allows insertion of non-pointer entries.
Do not disallow an ENTRY value of NULL.
(hash_insert): This is now just a thin wrapper.  Call hash_insert0.
* lib/hash.h (hash_insert0): Declare.