improve feedback if files are invalid symlinks

[Karl-Philipp Richter]

For some reason some binaries (e.g. `/usr/bin/reset`) got replaced by symlinks pointing to the link itself which made `dpkg` fail with errors in the form of `unable to make backup symlink for './usr/bin/reset': No such file or directory`. It'd be more helpful if `dpkg` would explain this issue since it's unlikely that a symlink to itself is intended.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: dpkg 1.18.2ubuntu5.1 [modified: usr/bin/dpkg-deb]
Uname: Linux 4.3.3-040303-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
Date: Fri Mar 4 00:13:42 2016
InstallationDate: Installed on 2015-04-20 (318 days ago)
InstallationMedia: Ubuntu-Server 14.10 "Utopic Unicorn" - Release amd64 (20141022.2)
ProcEnviron:
 TERM=screen
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: dpkg
UpgradeStatus: Upgraded to xenial on 2016-03-03 (0 days ago)
modified.conffile..etc.cron.daily.dpkg: [deleted]