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/mysql

@@ -6,10 +6,8 @@
 caption = MySQL I/O
 recv.metric = mysql.status.bytes_received
 recv.color = green
-recv.label = recv
 sent.metric = mysql.status.bytes_sent
 sent.color = blue
-sent.label = sent
 
 [mysql_keys]
 caption = MySQL keys