Merged from trunkk 5205:5212

Merged from trunkk 5205:5212

Share the same config instance between portagetree and portdbapi
instances since clone isn't needed here. Also, trigger population of
virtuals prior to cloning  in emerge.  This reduces the number of times
the virtuals are populated from 3  to just 1.

Enable FakeVartree to populate the virtuals so that the vdb doesn't have
to be  scanned separately.

portage_versions lacks a header or copyright notice :x

Catch an exception instead of using a redundant os.access call.

A quick docstring for vercmp.

s/zip/izip/  Thanks to Brian Harring for the suggestion.

s/zip/izip/ some more

svn path=/main/branches/prefix/; revision=5225