fixing "port" message

no args=query only
1 float arg=set new port
else = error
use pd_error() when possible
make REUSEADDR/REUSEPORT optional

by default we REUSEADDR is enabled and REUSEPORT is disabled,
but they can be set/queried via "reuse{addr,port}" messages.
fix deletion of removed nodes in notifier_update()

original code forgot to set the "last" variable