ls: fix columnar output. Closes 8731

ls: fix columnar output. Closes 8731

In coreutils/ls.c, 1.19 introduced commit
2f7d9e8903029b1b5e51a15f9cb0dcb6ca17c3ac, removing the variable tabstops and
hard coding the column separation to 2 characters, but was not done correctly.
The column_width assumes a gap of 1 character, so the computed number of
columns exceeds the terminal width when many small files are encountered.

A minor problem but surprisingly annoying.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>