mtr-packet: packet customization options (size, fill, mark, tos)

mtr-packet: packet customization options (size, fill, mark, tos)

Added send-probe arguments to customize the packet size,
the bitpattern which fills the packet, the IP "type of service"
and the Linux routing mark.  These arguments are documented
in the mtr-packet man page, and have been hooked up to mtr's
commandline parameters.

Internally to mtr-packet, the code is now more consistent about
using errno to return error codes.

Added instrutions for building on Windows to the README.

Fixed the Makefile to build the man pages with BSD make, for FreeBSD.
BSD make lacks the GNU make extension for '%' wildcard matching.

Added a helper program for testing which listens for an ICMP
packet with a particular sequence and prints information about
that packet.