ip2str: introduce changable address separator

This patch make change address string separator by "v4sep" or "v6sep" in config
file, because graphite uses `.' as path separator and statsd uses `:' as path
and value separator. Now I am testing ulogd and statsd (actually statsite) using
ulogd config:

  stack=ct1:NFCT,ip2str1:IP2STR,sprint:SPRINT
  [ct1]
  # in event mode, NF_NETLINK_CONNTRACK_DESTROY only
  event_mask=4
  [ip2str1]
  v4sep="_"
  v6sep="_"
  [sprint]
  form="myrouter.<orig.ip.daddr.str>.<orig.ip.protocol>.(<icmp.type>|<orig.l4.dport>|unknown).<orig.ip.saddr.str>:<orig.raw.pktlen> + <reply.raw.pktlen>\|kv\n"
  proto="tcp"
  dest="8125@192.168.1.1"