mtr-packet: MPLS decoding and local UDP port usage

mtr-packet: MPLS decoding and local UDP port usage

mtr-packet will decode any MPLS labels embedded in an
ICMP reply which results from a probe, and report those
labels with the probe reply.

When sending a UDP probe, a local port can be specified
for probe origination.  In the same way that the legacy mtr
code found a location to store a unique identifier for the probe,
we'll use the destination port, the local port or the checksum
field, depending on what probe arguments have been specified.

Both MPLS and local UDP port options have been documented in the
mtr-packet man page.

Update the SECURITY documentation to reflect mtr-packet, and did
minor copyediting in the README.  Also, update my email address
in AUTHORS.