Fix flag corruption due to mismanaged zero refcount.

r1760 updated the hash functions that Penn used. Unfortunately, two
locations in the flag code assumed that fc_hash(n->cache->zero) would
always evaluate to 0. Since this is no longer true, use fc_eq() to check
for equality with flagset zero.

git-svn-id: https://pennmush.googlecode.com/svn/trunk@1767 ba372814-4f39-11de-9ad6-1127a62b9fcd