nabstime.c is about to be removed, but timeofday() isn't related to
the rest of the functionality therein, and some find it useful. Move
to timestamp.c.

Discussion:
    https://postgr.es/m/20181009192237.34wjp3nmw7oynmmr@alap3.anarazel.de
    https://postgr.es/m/20180928223240.kgwc4czzzekrpsid%40alap3.anarazel.de