Merged from trunk 5284:5294

Merged from trunk 5284:5294

For bug #157897, handle the case of multiple versions within a single
package slot have been pulled into the dependency graph.  If possible,
the package that has already been pulled in will be reused.  Otherwise,
an error message will advise the user to mask an unwanted version.

handle --cols when displaying blockers (SpankMan)

For bug #157897, add slot collision handling in depgraph.create(), since
create() is called from places other than select_dep().

Update the comment about depgraph.pkg_node_map since it now maps all
nodes added to the graph.

Use depgraph.pkg_node_map so simplify comparison against existing nodes
at the  beginning of depgraph.create().

Fix the --onlydeps conditional logic in depgraph.create().  These aren't
real nodes and shouldn't be added to the digraph, etc...

Remove redundant priority handling since that's always handled in
select_dep() for nodes that have parents.

Remove pointless unpacking of the parent node.

Make --columns blockers output more consistent when --quiet is not
enabled.

svn path=/main/branches/prefix/; revision=5295