Merged from trunk 5238:5247

Merged from trunk 5238:5247

Add use.force support to dep_check for repoman especially. Thanks to
Flameeyes for reporting and tesing.

Use portage.isvalidatom() for atom validation in repoman.

Make isvalidatom() (and thus repoman) reject atoms containing |()
characters.  Thanks to Brian Harring for the suggestion.

Add ? to the invalid atom characters.

Clean up depstring/atom splitting and filtering.

For bug #157369, clean  when  cleanup has been triggered by changed
timestamps.  Also, allow FEATURES=keepwork to override the ebuild mtime
check.

This patch cleans up the blocker validation logic.  Unlike the previous
implementation, this one carefully considers each parent/block package
relationship in order to ensure that no unresolved blocks slip through.

Don't attempt to enforce merge order unless the blocker parent is
actually a "merge" node.

Escape a newline from the previous commit.

svn path=/main/branches/prefix/; revision=5258