unicorn horn vs deafness

unicorn horn vs deafness

Implement the suggestion that applying a non-cursed unicorn horn can
cure deafness like other similar troubles.  Also, applying a cursed one
can cause deafness, although I made the chance be half of what it is
for the other troubles since they tend to be more significant.

This is entry #2 on the bugzilla list, but I haven't figured out how to
update that yet.