Add $history_remove_dups option to remove dups from history ring.

Add $history_remove_dups option to remove dups from history ring.

When set, duplicate entries will be removed from the history ring when
a new entry is added.  The duplicate removal rearranges the history
ring such that created empty slots are right after the "last" position
in the ring, preserving the most history.

Rewrite the next/prev functions to take into account that blank slots can
now be in the middle of the array.