Convert valid IPv6 addresses to ip6.int PTR values.  Convert valid
IPv4 addresses to in-addr.arpa PTR values.  Anything not valid is
simply printed as is.  Handles :: notation and embedded IPv4
addresses.  If the address is followed by /n, the PTR is truncated
to n bits.

- Jean-Yves Lefort
jylefort@brutele.be
