Bug#220550: base-installer: should take into account that deboostrap is not idempotent

[Sebastian Ley <sebastian.ley@mmweg.rwth-aachen.de>]

Package: base-installer
Severity: important
Tags: d-i

Debootstrap is not idempotent, it will fail if told to bootstrap a
partition where already an instalation of linux resides.

Base-installer should be aware of the fact and try to rule out problems
before debootstrap runs.

Idea:
Try to detect if there are offending files in /target, if yes, tell the
user that an installation was found. He may then choose to keep it (exit
base-installer with a note to run partconf again) or to trash it (rm
-rf)

-- System Information:
Debian Release: testing/unstable
Architecture: i386
Kernel: Linux coyote 2.4.22-1-k7 #5 Sat Oct 4 14:11:12 EST 2003 i686
Locale: LANG=de_DE@euro, LC_CTYPE=de_DE@euro