hash: deprecate poorly-named hash_insert0: use hash_insert_if_absent

hash: deprecate poorly-named hash_insert0: use hash_insert_if_absent

* lib/hash.c (hash_insert_if_absent): Rename from hash_insert0.
Add a sentence to the comment.
(hash_insert0): New function that simply calls hash_insert_if_absent.
* lib/hash.h (hash_insert_if_absent): Declare it.
(hash_insert0): Add deprecation attribute.
(_GL_ATTRIBUTE_DEPRECATED): Define.
* lib/di-set.c (di_set_insert): Use hash_insert_if_absent,
not hash_insert0.
* NEWS: Mention it, even though it's not really an incompatible change
Prompted by a question from Matthew Booth <mbooth@redhat.com>.