Pretty-print values in the PCP DynamicMeter code

Several improvements to the way values are displayed in the
PCP platform DynamicMeter implementation:
- handle the initial 'caption' setting as with regular meters,
  this required a new meter callback because we no longer have
  just a single meter caption for the DynamicMeter case
- if no label is provided for a metric in a configuration file
  use the short form metric name as a fallback
- honour the suffix setting in the configuration file
- convert metric values to the canonical units for htop (kbyte
  and seconds), and use Meter_humanUnit when it makes sense to
  do so.

Also improves the handling of fatal string error messages in a
couple of places, thanks to BenBE for the review feedback.

pcp/meters/tcp

@@ -13,7 +13,6 @@ active.color = blue
 active.label = act
 syn.metric = network.tcpconn.syn_sent + network.tcpconn.syn_recv + network.tcpconn.last_ack
 syn.color = cyan
-syn.label = syn
 wait.metric = network.tcpconn.time_wait
 wait.color = red
 wait.label = tim