Merged from trunk 4740:4753

Merged from trunk 4740:4753

Filter empty elements in cumulative env-update variables.  Thanks to
Brian Harring for this patch from bug #151694.

Fix CONFIG_PROTECT so that is works with symlinked directories for bug
#151502.

Make empty digest-* files 0 bytes instead of 1 byte.  Thanks to Sven
Wegener for reporting this.

Eliminate an unnecessary string concatenation.

Add more dependency priority levels for merge order calculations.
Previously,  all installed deps were categorized at the same level with
other soft deps.  Now there are 3 priority levels for soft deps:
satisfied buildtime, satisfied runtime, and other.

Remove the unused system slot from DepPriority.

Fix --usepkg --newuse logic so that it doesn't throw a KeyError if
there's no matching ebuild in the tree.

Add __weakref__ to __slots__ so that weakref support isn't disabled.

Don't initialize __weakref__ in the DepPriority constructor.

All packages to be added to the world favorites via --noreplace, even
when --ask is enabled.  Thanks to leio for reporting this issue.

Explicitly set the priority when a node is added to enforce merge order
for blocker avoidance.

Handle unresolvable blockers properly for bug #151822.

svn path=/main/branches/prefix/; revision=4754