Make selection consistent over line breaks.

Currently, selection is expanded to the end of the line over line breaks only in
regular selection mode, when the line in not empty and when going down and/or
right. This covers all the cases including word selection mode, with the
exception of rectangular selection because it would make this mode too rigid.
This adjustment is made in selsort so I renamed it to selnormalize to better
reflect what it does now.

Signed-off-by: Roberto E. Vargas Caballero <k0ga@shike2.com>