mtr-packet: IPv6 support

mtr-packet: IPv6 support

mtr-packet can now send and receive ICMPv6 probes.

We now determine the source address for an outgoing probe by
opening a UDP socket to the destination, though no outgoing packets
are sent through this UDP socket.  IPv6 made this necessary, but
it now occurs for IPv4, too.

mtr-packet now uses sockaddr_storage for passing around addresses,
rather than sockaddr_in, as it can work for either IPv4 or IPv6.

To improve the maintainability of the code, the packet construction
and interpetation has been moved from probe_unix.c to construct_unix.c
and deconstruct_unix.c.

The way that Windows reads from the command stream has been changed to
avoid the possibility of hanging in a Sleep without an active read
of the command stream.

"send-probe" will now respond with "no-route" or "network-down"
when those conditions apply.

The test code has been moved to a test directory and split into
the following modules:

probe.py - tests for sending probes
cmdparse.py - tests for command parsing
mtrpacket.py - infrastructure for testing mtr-packet

The mtr-packet man page has been updated to describe ip-6 options
and now has an example of tracing a route to a remote host.

The mtr-packet network initialization has been split into two halves
to minimize the operations which occur with elevated privileges.