Accept "+infinity" in date and timestamp[tz] input.

The float and numeric types accept this variant spelling of
"infinity", so it seems like the datetime types should too.

Vik Fearing, some cosmetic mods by me

Discussion: https://postgr.es/m/d0bef637-2dbd-0a5d-e539-48243b6f6c5e@postgresfriends.org
