previously, if "listen-on-v6" was set to "none", then every
time a scan saw an IPv6 address it would appear to be a new
one. this commit retains all known interfaces in a list
and sets a flag in the ones that are listening, so that
configured interfaces that have been seen before will be
recognized as such.
as an incidental fix, the ns__interfacemgr_getif() and _nextif()
functions have been removed since they were never used.
6df5cf1ee6