Added mtr-packet subprocess

Added mtr-packet subprocess

The mtr-packet tool isolates the raw socket sending/receiving of
packets from the mtr user interface.  By isolating the socket
interactions to a separate process, we can be sure that any security
flaws in the user-interface code doesn't expose a raw socket
interface to an attacker attempting to escalate privileges.

This is a bare-bones implementation, only support ICMP,
only support IP version 4, and missing many of the probe customization
features available in mtr.  It will require some more work to reach
feature parity with the current mtr implementation.  But it's a start.

The include mtr-packet man page explains the protocol format
used to communicate with this new process.

Included is an automated test for mtr-packet, implemented using
Python's unittest module.  Though the code actually being tested
is implemented in C, Python make it easy to write test cases.
'make check' will test the current build.

An alternate code-path for Windows is included in the mtr-packet
tool.  The mechanism for sending and receiving network probes
is significantly different for Windows, as compared to Unix-like
operating systems, but the interface provided by mtr-packet is
the same.  'make dist-windows-bin' will make a Windows binary
distribution.  A Cygwin build environment is required, but the
resulting binary distribution doesn't require that Cygwin be
already installed.

Tested on:

Ubuntu 16.10, FreeBSD 11.0, MacOS 10.12.1 (Sierra), Windows 7

Since the code changes are significant, more esoteric operating
systems may require changes.