Convert the HASH to be usable for either string or unsigned int keys,
so that a uid hash can be added for imap.

To keep hash-usage code disruption to a minimum, this introduces new
create/insert/find/delete functions for the int hash, but keeps the
old function names for string keys.

This implementation makes the key a union.  It may have been a better
idea to introduce a whole new structure, but this way allows minimum
changes to and maximum reuse of the existing hash code.