fixed time-histogram x label off-by-one
changed time histogram x decoration style to stop
removed superfluous span_t enum
non-tcp packets contribute to time histogram

Now, when a packet without a TCP segment in it is received, it is passed to
time histogram with the F_NON_TCP flag.  The histogram then records it as a
portless packet.  Additionally, time histogram view now displays the non-tcp
packets as a black-outlined empty bar.