astraceroute: Support binding to a specific IP address.

In a situation where the IP that you wish to use as the source address
differs from the interface it needs to be sent from, it is now possible
to manually specify an address to bind to whilst still being able to
specify the physical interface to send from.

[Fixed up whitespace, minor coding style and added man-page entry --DB]

Signed-off-by: Oliver Smith <oliver@zerolag.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>