IPv6 dual-stack support

This patch implements full IPv4-IPv6-dual-stack support for umurmur. It
introduces some new configuration and command line options:

  - The '-A' and '-B' options which work similar to the old '-a' and
    '-b' options but for IPv6.
  - The 'bindaddr6' and 'bindport6' options, which too, work like their
    IPv4 counterparts.

By default, the IPv6 implementation listens on all addresses and the
default port 64738, just like the IPv4 part. If your system does
support IPv6 and you haven't setup a proper IPv6 firewall, now is the
time to do it. In the meantime you could simply listen on the loopback
device (::1), if you'd like to 'disable' IPv6 functionality.

Note that since the implementation is fully dual-stack, umurmur will
simply warn you if either IPv4 or IPv6 is unavailable but it will still
run fine with no further action is required (unless of course you want
to use one of the unavailable protocols, which would mean you'd have to
enable it in your kernel configuration).