Vertical spacing changes around tables in man_term.c rev. 1.194
and tbl_term.c rev. 1.66 cause quite a bit of churn, unfortunately.
This commit cleans up most of it.

Since resetting of offsets works quite differently in man(7) and mdoc(7),
test table centering in an mdoc(7) document as well.
Related to tbl_term.c rev. 1.55.