Improve psql's internal print.c code by introducing an actual print API.

Improve psql's internal print.c code by introducing an actual print API.
Provides for better code readability, but mainly this is infrastructure changes
to allow further changes such as arbitrary footers on printed tables.  Also,
the translation status of each element in the table is more easily customized.

Brendan Jurd, with some editorialization by me.