Remove 'active_tls_port' support

Unencrypted TCP connections and encrypted TLS connections are now always
handled on the same port. This simplifies things a bit and should make
it easier to add IPv6 active mode support.
vars.c: Fix var_parse_ip[46]() to handle NULL
listen.c: Add IPv6 support

It's not actually used yet, since hub.c is responsible for telling
listen.c to get into active mode and which IP version to use, but that
part hasn't been updated yet.
doc.h: Remove active_tls_port

I've not updated doc/ncdc.pod.in yet, I'll do a complete review of that
once all the IPv6 stuff is done.