Do not handle vertical lines as additional tbl(7) columns,

Do not handle vertical lines as additional tbl(7) columns,
instead save their properties with the following column.
This simplifies layout parsing and saves a lot of code
related to column handling.

At output time, print all white space and vertical lines
separating columns before printing the following column,
and none after printing the preceding column, considerably
simplifying white space handling and width calculations.

No functional change, but it saves 150 lines of code,
and it allows the next patch to tbl_term.c, tbl_literal().

"Please check them in and I'll look into them later!" kristaps@