libsmartcols: use standard deviation to optimize columns width

libsmartcols: use standard deviation to optimize columns width

The standard deviation together with mean (average) of the data width
can be used to detect "problematic" columns and to calculate optimal
width.

The idea is to sort column by avg+deviation and start columns width
reduction from the column with the most wide and variable width. The
width reduction is also optimized by 68–95–99 rule (aka empirical
rule, avg+n*deviation; where n={1,2,3}) to cover 95% or 68% data in
the column.

The disadvantage is we need to link libsmartcols with -lm (math) due
to sqrt() function.

Signed-off-by: Karel Zak <kzak@redhat.com>