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