Merged from trunk 5990:5995

Merged from trunk 5990:5995

Move reverse NEEDED cache into CACHE_PATH and use a more descriptive
name (genone)

extend check for internal references, should remove all libs that are
only used internally now (genone)

Move preserve-libs and collision-protect code from treewalk() into their
own functions (genone)

set.remove() returns None, so can't use it in a function call (genone)

svn path=/main/branches/prefix/; revision=6079