introduce "epoch" as a new base time reference, meaning timestamp 0.

introduce "epoch" as a new base time reference, meaning timestamp 0.
This disambiguates between some "at-style" time specs:

11111111 yields an error message:

did you really mean month 1111111?

and

19711205 denotes Dec 12, 1971 and not the timestamp.

The patch allows to write:

epoch+11111111s or epoch+19711205s

-- Peter Stamfest

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@2147 a5681a0c-68f1-0310-ab6d-d61299d08faa