Use case: I ITP etckeeper (https://etckeeper.branchable.com/) which I 
frequently use on Debian. For fully automatic operation, it requires 
pre-install and post-install hooks, e.g:

/etc/preremove/0p_000_etckeeper_pre-install.sh
/etc/postinstall/zp_zzz_etckeeper_post-install.sh

This patch adds the missing functionality to run the pre-install hook. 
It is limited to /etc/preremove/0p_* because there is possibly no use 
case for /etc/preremove/zp_*.

'class Perpetual0RemoveFindVisitor' is borrowed from postinstall.cc and 
modified. It still uses the ugly pre-C++11 hack to disable copy-ctor and 
operator=. Possible refactoring like merging all 3 mostly similar 
visitors into one (or if C++11 is now allowed, use lambda functions 
instead) are left for later.

-- 
Regards,
Christian