rrdcreate: support scale suffix for heartbeat/steps/rows

This feature allows this opaque create specification:

  rrdtool create power1.rrd \
    --start 0 --step 1 \
    DS:watts:GAUGE:300:0:24000 \
    RRA:AVERAGE:0.5:1:864000 \
    RRA:AVERAGE:0.5:60:129600 \
    RRA:AVERAGE:0.5:3600:13392 \
    RRA:AVERAGE:0.5:86400:3660

to be expressed with more understandable durations:

  rrdtool create power2.rrd \
    --start 0 --step 1s \
    DS:watts:GAUGE:5m:0:24000 \
    RRA:AVERAGE:0.5:1s:10d \
    RRA:AVERAGE:0.5:1m:90d \
    RRA:AVERAGE:0.5:1h:18M \
    RRA:AVERAGE:0.5:1d:10y

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
rrdcreate: improve scale support

- Fix missed diagnostic for negative values accepted by strtoul.
- Add detailed diagnostics for parsing failures.
- Add diagnostic when duration-based RRA steps (rows) are not multiples of
  PDP interval (RRA span).
- Fix row count when PDP interval not one second.
- Extend documentation to special-function RRA example.
Merge pull request #468 from pabigot/enh-create-with-dur

rrdcreate: support scale suffix for heartbeat/steps/rows