Fix ls -l alignement with new locales

Latest update of locales introduced abbreviated month that follows the regionale
rules meaning that they can be of variable length instead of being arbitrary
truncated to top 3 characters.

To fix alignement, ls now computes the visible length of the abbreviated month,
pads the shorter month with spaces in order to make sure everything is properly
aligned

Reviewed by: ache, ed, jilles
Differential Revision: https://reviews.freebsd.org/D4239