In config.setcpv(), use LazyItemsDict to implement lazy evaluation of

In config.setcpv(), use LazyItemsDict to implement lazy evaluation of
PORTAGE_IUSE. The PORTAGE_IUSE value is lazily evaluated since re.escape()
is slow and the value is only used when an ebuild phase needs to be executed
(it's used only to generate QA notices). Thanks to Marat Radchenko
<slonopotamusorama@gmail.com> for identifying this performance issue and
submitting the initial patch which used a memoization approach instead of
lazy evaluation.

svn path=/main/trunk/; revision=12673