fix a handful of problems in rrdcopy

rrdcopy is a great start, but as it is in git, it isn't actually capable of migrating a Munin "normal" rrd to a "huge" rrd.  These changes fix rrdcopy so that it can copy normal->huge and huge->huge (normal->normal and custom haven't been tested).  I also added a bit to the documentation that might be obvious to some, but would have saved some time on IRC.