column(1): add -l flag

the '-l <tblcols>' flag limits the number of columns that column(1) will
produce in -t mode. this is syntax-compatible with the same option in
util-linux's column(1), but due to existing differences between the two
implementations, it's not semantically compatible.

as a side-effect, fix a pre-existing bug where empty fields could cause
incorrect output:

% echo ':' | column -ts:
(null)

while here, also fix a couple of minor existing issues.

Reviewed by: des
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50290