Allow multislot packages to be added to the world file

Allow multislot packages to be added to the world file
via --noreplace. Having these atoms in the world file
will trigger recommendations to run emaint in some
cases, like when running `emerge -e world`. The atoms
need to be in the world file to prevent multislot
packages from being removed by --depclean though.
(trunk r8436)

Bug #198129 - Prevent SLOT atoms like  sys-devel/binutils:0
from being inappropriately recorded in the world file when
USE=multislot is enabled.
(trunk r8437:8439)

svn path=/main/branches/2.1.2/; revision=8440