mtr-packet: TCP and SCTP probes

mtr-packet: TCP and SCTP probes

Added support for generating probes using TCP or SCTP, for both
IPv4 and IPv6.  These protocols require that we create a unique
socket for each probe and watch for either a TTL expiration of
the initial packet sent during a connection attempt from that
socket or for socket connection success.

We now allocate a unique port number or ICMP sequence ID when
a probe is created, rather than using the command token for this
purpose.  This relieves the calling application of the burden
of picking sensible command token values, and allows command
tokens values greater than 16 bits.  However, the existing mtr
code continues to use the same command tokens values it has
previously used as port numbers, so there is no difference when
the calling program is mtr.

Split mtr's command pipe handling out from net.c and into cmdpipe.c
in the interest of future maintainability.

Split probe.py's simple probes out from the individual protocol
test cases and into common code which can be used by many
protocol tests.