Make --skipfirst handle missing dependencies by dropping the merge tasks

Make --skipfirst handle missing dependencies by dropping the merge tasks
that have missing dependencies. This involves creating an entirely new
depgraph each time that a missing dependency is discovered. This isn't
the most efficient approach but it's simple and it works well.

This new behavior can server as a fix for bug #12768 by calling emerge
--resume --skipfirst each time that emerge exits unsuccessfully. This
was possible before, but now packages with unsatisfied dependencies
will be properly identified and dropped from the list.

svn path=/main/trunk/; revision=10478