These functions retrieve or set the key for a hash entry. The key
is stored in the hash entry so that it may be compared when inserted,
looked up etc. entry must have the RHASH_HEADER as its first
member.
Note that it is not necessary to use rbhash_entry_setkey(3) to set the
key prior to insertion into the hash table. rbhash_insert(3) takes
the key as an argument and sets it within the insert function.
