noip.c, noip.1: Multiple address family support.

noip.c, noip.1: Multiple address family support.

Abstract out all of the address-family-specific hacking into a
collection of utility functions.  Now, with a little luck, adding
additional address families later will be straightforward.

The main casualty is the notional support for arbitrary netmasks, though
in fact they never worked correctly.  They've now been silently dropped:
the new parser simply refuses to try.