Optimize LinkageMap to use tuples of device and inode numbers from stat

Optimize LinkageMap to use tuples of device and inode numbers from stat
calls, instead of paths from realpath, as unique keys for identification
of files. This is the same approach used by dblink.isowner() for cases
in which path comparison fails due to symlinks.

Thanks to Lucian Poston <lucianposton@gmail.com> for submitting this patch
(along with the missing-rebuild package set which I haven't merged yet). These
patches are hosted in the following location:

http://repo.or.cz/w/revdep-rebuild-reimplementation.git?a=tree;h=refs/heads/rc3;hb=refs/heads/rc3

svn path=/main/trunk/; revision=11447