Fix make_timestamp[tz] to accept negative years as meaning BC.
Previously we threw an error. But make_date already allowed the case,
so it is inconsistent as well as unhelpful for make_timestamp not to.
Both functions continue to reject year zero.
Code and test fixes by Peter Eisentraut, doc changes by me
Discussion: https://postgr.es/m/
13c0992c-f15a-a0ca-d839-
91d3efd965d9@2ndquadrant.com