rrd_scaled_duration: refactor as utility function

Move the infrastructure supporting expression of counts and durations by
suffixed integers into librrd where it can be used by other programs.
Clean up the documentation to remove redundancies.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
rrd_fetch: improve resolution-related parameters

The interface now accepts scaled durations (e.g., 5m) for the
resolution.

A new option automatically performs the start time alignment discussed
in the documentation.
librrd.pod: fix syntax errors
Merge pull request #474 from pabigot/enh-fetch-duration

Use new duration interface in rrdfetch