In .Bl -column, if some of the column width declarations are given

In .Bl -column, if some of the column width declarations are given
right after the -column argument and some at the very end of the
argument list, after some other arguments like -compact, concatenate
the column lists.
This gets rid of one of the last useless FATAL errors
and actually shortens the code by a few lines.

This fixes an issue introduced more than five years ago, at first
causing an assert() since bsd.lv mdoc_action.c rev. 1.14 (June 17, 2009),
then later a FATAL error since mdoc_validate rev. 1.130 (Nov. 30, 2010),
and marked as "TODO" ever since.